The reason more people didn't die is because much attention was focused on not dying. As much as WW I Generals get lambasted for thoughtlessly sending men to the slaughter (which did happen in isolated situations), the reality is that most of them were feverishly working on ways to keep their men alive while killing the enemy.

Trenches are a great example. Trenches are often used as a symbol of the horror of fighting in WW I. And while living in one wouldn't be great (although the German trenches could be quite cozy), you were remarkably safe in one for the most part. Some of the highest % casualty figures in WW I happened right in the beginning when most had no understanding of what fighting in the open would be like. All sides quickly learned and dug in (particularly on the Western Front). In some situations (particularly with the Germans) the trenches were dug with concrete enforced bunkers deep underground. Practically impervious to artillery. And the strategy developed to have multiple parallel trenches that were connected. So often the majority of men were either in deep bunkers or in a trench further back during those horrible WW I artillery bombardments, then they'd rush up to man their positions when the infantry attacked. In sum, trenches were great at keeping people alive.

Tactics also rapidly developed during WW I, fully embracing open order infantry tactics as the fighting went on, rather than more closed order formations that were seen at the very start of the war. Spread out, hug the ground, etc... During assaults, great care was typically made to limit casualties as much as possible. This includes incredibly intricate artillery barrages, creeping barrages, feints, smoke screens, chemical weapons, etc... to try and give the attacking forces cover. They didn't always work, but they often did.

As for your comment about rifle and MG fire, the vast majority of casualties in WW I were caused by artillery, not infantry rifles or MG's. The overwhelming majority of bullets fired in a war (any war) miss. Most are shot w/out aiming or maybe even seeing the enemy. MG's are as much about suppression than they are mowing down a group of men (although of course they can do that too).

I'd also add that humans by and large really do not want to die, and they act accordingly. Most soldiers in an army are brave, but they're not looking to win a MoH. They want to do their job, support their buddies, and get home. Most officers understand this and try to avoid breaking their men (or getting shot in the back). If an MG opens up, most men will go to ground, not charge it heroically (see prior point about MG and suppression). Conventional wisdom in "modern" warfare is that when a unit hits 30% casualties, it is combat ineffective and should be rotated out. Of course there are many examples of units exceeding this and continuing to fight, but it's usually out of desperation. When they can, most officers would rotate a unit out if they took that many casualties. Again, soldiers are normally brave and do their duty, but they aren't Rambo's.

That leads to my last thought about WW I. While living in trenches and going over the top were not great situations to be in, those moments for most soldiers were incredibly rare. On the Western Front units were regularly rotated back to the rear for R&R or less onerous duties before spending relatively short periods in the front line trenches. Senior leadership generally understood their men had breaking points, and they did their best to not push it.

None of this is to say that fighting in WW I wasn't so bad. If I had to fight in any war in history, WW I would be near the bottom.

Edit: Wow, thanks for all the awards!

TLDR: The vast majority of the time soldiers were not involved intense fighting and most soldiers were not at the front at any given time either because of rotations or because of their role in the military. Casualties added up to become quite significant from all these little raiding parties, artillery shelling, and the occasional sniper, but each day in this type of war a regiment might see only 2 or 3 casualties. When assaults happened they were quite deadly, but not that many troops were at the tip of the spear for it.

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First, we need to acknowledge that casualties in WW1 were incredibly high.

8% of British Soldiers were killed, 15% of German Soldiers were killed, 16% of French Soldiers were killed. To fully understand that number we have to understand that many many more were wounded or suffered from illness. Germany had 19 million cases of hospitalized illness or wounds. That obviously runs the whole gambit from people who are permanently disabled or may even die decades earlier than normal to people who fully recover at the end of the week, but it goes to show that much more than 15% of German soldiers were affected. 

Most soldiers in WW1 became injured or sick during their service to the extent they had to be taken off the front-line (not common colds and splinters, but not necessarily life altering injuries either) The significant majority of these illness and injuries were not serious but there was certainly more serious injuries than deaths. 

It is impossible to calculate what percentage of WW1 vets became disabled as the definition varied between countries, but to give a sense two million British soldiers (between 20-25%) returned with some degree of war-related disability (more than double deaths), as did 200,00 Americans (slightly less than double deaths). The French too had more disabled than killed. Its important to note that disability could have meant losing a finger or having a bad leg the rest of your life, not amputations but this still needs to be taken into account. About 200,000 British soldiers suffered from very severe disability, generally meaning loss of a limb, severe facial disfigurement, and blindness, . 

Psychiatric casualties are almost surely underreported because of stigma at the time but 24% of US troops (who were there for a much shorter time) became psychiatric casualties at some point in the war. The extent to which this caused permanent PTSD and trauma is unknown but likely massively underreported. So in all your chance of becoming a casualty was not 1 in 10 but more like 1 in 2 with maybe 1 in 5 suffering life-long effects from their injuries and illness. 

Second, we should acknowledge that most of War is not fighting

One popular historian estimates that British spent about 25% of the time on the front (forward and support trenches), 30% of the time near the front and 45% of the time away from the front. It is also true that much of the front, much of the time was not terribly active. The winter months tended to see a lot less fighting and even when there was a lot of fighting this tended to be concentrated in certain areas with many troops standing unengaged in others. Only in 1914 and 1918 was there a state of general offensive on the Western Front. The great battles of 1916 and the short-lived Nivelle offensive in 1917 did see a significant portion of troops engaged, but most of the time troops were engaged in a much lower intensity sort of war.

Combat losses also tend to be concentrated in combat units. In WW1 armies were beginning to build up large logistical trains, although not quite to the extend that they have today. If we take the American army as an example only 28% of their total forces were engaged as combat troops, and only 53% of their troops in Europe were engaged as combat troops. See McGrath, The Other End of the Spear for more details 

Those combat troop numbers also includes a significant portion dedicated to artillery. Artillerymen were still very much in danger but somewhat less than the infantry. A rough back of the envelope calculation shows that about a quarter of British troops were in the artillery (this varied overtime as the number increased throughout war), but only 7% of British casualties were artillerymen. While I wouldn't try to draw an exact ratio from that number the point is that the much of the army were not grunts in the trenches, and that grunts in the trenches were much more likely to die than the average British soldier. 

This also lead us to our first important limitation on casualties. 

The US army has estimated that a unit that suffers over 30% of casualties is combat ineffective (in that it will fight but not very well and generally not in offensive operations) and a unit that suffers over 50% casualties is basically destroyed. This obviously varies with training, belief in the cause, and the situation. Units will fight to the last man if they don't think they have any other option (i.e surrounded by an enemy that doesn't takes prisoners), but there is breaking point. This is both because the morale of such troops is so low that they'll refuse to follow orders and because they truly do not have the numerical strength to resist attacks and have probably lost significant amounts of equipment. Furthermore, casualties among officers in the field tend to be higher as they are identified and shot at and this further destroys unit cohesion and command ability. While looking at the raw data might suggest that operation only had a casualty rate of 10%, the casualty rate for the combat infantry engaged in the principal areas of action is likely significantly higher and often approached the rate to which those units were rendered combat ineffective and were forced to either break off the attack or the defender retreated. Particularly egregious examples would be on the First Day of the Somme were a dozen or so battalions suffered 70 to 90% casualties (Newfoundlanders, 10th West Yorkshire and 8th York and Lancaster) and whole division suffered casualties in the 30-50% range within day or two. There are many many instances were specific assaults led to the type of disastrous casualties that one might expect from a WW1 battle, but what we have to understand is that most of the time troops aren’t in a battle, and even when they are the number of those troops engaged in the principal action is quite small.

Now that we have gone over everything that might cause us to underestimate casualties lets go into things that did actually mitigate casualties.

In the 19th century, disease and infected injuries killed significantly more soldiers than battlefield deaths. More than twice as many soldiers died from disease than battle in the American Civil War and the Napoleonic Wars (although records are more sketchy for the later) and the significant majority of casualties in the Crimean War had been caused by disease. In World War I more soldiers died on the battlefield than from disease and if it wasn’t for the 1918 Influenza Pandemic this would have been a significant majority. This was not true for the US because its soldiers missed most of the war but not the Pandemic, but even here the ratio was just over 1 to 1 rather than the 2 to 1 ratio of the Civil War. While proper antibiotics did not exist yet and trench life was often unsanitary medical understanding had advanced significantly. The Western Powers and to a lesser extent Germany had access to vaccines and drugs that curbed Malaria, Typhus, and Tetanus some of the traditional big killers. Tuberculosis became screened for, soldiers were regularly deloused, there was a significant effort although not always successful to bring clean drinking water to the front. The result was a dramatic decrease in infectious deaths.

While treatment of injuries was a far cry from what soldiers get today, it was much better than what had come before. The revolution in medicine in the 19th century and the considerably greater degree of logistical organization of these armies meant that they had larger and far more advanced medical services than previously. A full understanding of the importance of sterilized medical instruments was becoming known and deaths from amputations became the exception rather than the norm. This also reduced casualties significantly.

As for the battlefield, why were casualties comparatively low? As explained earlier, in the case of actual contested trench assault they weren’t. The seemingly low ratio of casualties can be explained that the number of troops actually going over the top in an area was fairly low as proportion of overall troops and that these assaults happened fairly infrequently. The staggering casualties taking by the British at the Somme occurred mostly during the first 15 minutes over the top and shows just how devastating charging through no man’s land could be. Still the idea that the attacking force in a trench assaults were lambs to the slaughter is not true either. Tactics improved over time, but the general idea was that the artillery and suppressive fire from one’s own trench would help pin down the enemy force, and there was often plenty of natural cover in no man’s land that allowed troops to advance much of the way without being exposed to withering fire and to provide more accurate suppressive fire to support troops pushing forward. In many cases the enemy frontline was weakly held as a very large attacking force was concentrated on a not particularly fortified part of the line and the enemy often retreated rather than face a near certain death in hand to hand fighting. This generally did not lead to a catastrophic breach of the line as there tended to be plenty of reserve lines especially later in the war. So, soldiers in trench assaults tended to have a significant numerical advantage and used tactics that minimized their exposure. When this wasn’t true these assaults were indeed bloodbaths.

Don't rely on movies for your understanding of WW1 tactical realities, for starters.

Eg., the Western Front was the first large-scale fight in human history in which deaths from disease were much lower than those in combat.

Malnutrition? Most of the Allied soldiers were eating at least as well as they did at home and often better -- malnutrition was rife in ordinary working-class life then, which is why British working-class men were 4 inches shorter in 1914 than they are now.

And while offensives could produce very high casualties, they were relatively rare. Most of the time most of the front line was relatively "quiet", meaning there was only a steady trickle of losses from sniping, the odd trench raid, and off-and-on mortar and artillery attacks.

Trenches saved lives. It was when you got -out- of the trench that you were in real danger. "Hopping the bags" was the danger point. That's why the armies dug in in the first place.

The highest casualty rates on all sides were in the first couple of months of the war and the period from March-November 1918, both periods of open or semi-open warfare.

As for morale, the reason the war lasted so long and killed so many was that morale was usually quite high. These were armies of patriotic citizen-soldiers, with strong internal bonds and intense commitment to the national cause. That's why Russia's army cracked earlier than the others; most Russian peasants -didn't- have a strong sense of national identity at that time.