There are in fact two popular misconceptions at play here.

Firstly that the casualty rate in WWI was somehow abnormal compared to what passed before or has happened since, and secondly that the reason the casualty rate was excessive was because The Generals (tm) were too unimaginative, conservative, or just plain stupid to do anything about it.

So first of all the casualty rates were nothing that hadn't been seen before or hasn't been seen since. French casualties at Waterloo, for example, approached 60% and coalition casualties were around 20% in the same battle. Nor was Waterloo unprecedented by the standard of the time. In the next war, British average weekly casualty rates in Normandy in 1944 exceeded the average casualty rates of the first war by around a tenth.

No, the casualty rates were not remarkable. The absolute numbers appear shocking because for the first time in history it was possible to maintain an army of a million men in the field indefinitely. The high casualty lists are simply a function of the sheer scale of operations and the vast numbers involved - The Germans may have lost 40,000 men on the first day of the 1918 Spring offensive but they attacked with 74 divisions. That's more divisions that they had in the whole of western Europe in 1944

Its particularly galling for the English speaking world as never before, and never since, has a British army taken the role of the main combatant fighting the main body of the main enemy in a general European war. The casualties suffered on the first day of the Somme are widely regarded as a national catastrophe, but they are dwarfed by the losses the Soviets suffered when it was their turn to engage the main body of the main German army in WWII - they suffered tens of millions of casualties - ad that's military losses alone.

Its just plain old expensive in terms of blood and treasure to fight a general war in the age of high explosive and magazine-fed rifles.

As to The Generals (tm) - You will forgive my Anglocentric viewpoint here but the British Empire does best demonstrate the enormous learning curve that the all armies in WWI had to master. It is simply accentuated in the case of the British because they started with a comparatively tiny army, expanded drastically and went through a period of de-skilling, before attaining a fever-pitch of efficiency in late 1918.

Now, this war saw the introduction or first use of Dreadnoughts, mines, torpedoes, aircraft carriers, aerial reconnaissance, aerial artillery spotting, the fighter, ground attack aircraft, strategic bombing, armoured warafe, chemical warfare, the flame thrower, predicted indirect artillery fire, flash spotting and sound ranging, electronic warfare and electronic countermeasures, the introduction of squad-level automatic weapons, first widespread use of hand grenades etc. and I could go on!

Now, how is the demonstrable pace of technological advancement to be reconciled with the concept of old fuddy duddy General Melchets sipping wine at Chateaux 30 miles away from the front? Quite simply, it cant be.

As a case study, we can look at the use of hand grenades in the British army. The British first encountered Germans using grenades in October 1914. At first supplies of British grenades were limited and their use confined to engineer units. So “jam jar” grenades were improvised. By Spring of 1915, supplies were sufficient for specially trained ‘bomber’ units, 20 strong, to join with a Company on attack. By October of that year, all ranks were trained as bombers. By late 1916, each platoon had a specialist bombing section. By early 1917, platoons were organised with specialist section for bombing, rifle bombing, rifles, and Lewis gunners.

Another example is the use of the ‘leap-frogging’ tactic used to great effect by 1917. It required a brigade to send battalions in waves, 1st battalion to seize the first objective, then to be leap-frogged by the second battalion to seize the second object, and so on. Now at the time of the Somme, this was acknowledged as effective but far too complicated to be executed in battle. Yet by the battle of Arras in 1917, this was becoming standard practice.

Yet another example is the creeping barrage. It was suggested in 1916 that good results could be obtained if the infantry followed very closely behind a sequentially lifting barrage. At the time this was dismissed as foolishness and yet again, by 1917, this was standard practice for a set piece assault.

So I guess the TL;DR of this is that casualties were high because of the number of men and the scale of operations, and The Generals ™ were really coping with an unprecedented pace of technological change and doing so in the most difficult circumstances.

Hi there, there's always room for more answers on this topic, but this section of the FAQ may be of interest for you in the meantime:

http://www.reddit.com/r/AskHistorians/wiki/wwi#wiki_trench_warfare

While the main tactic of using a large amount of men to break the line of the other was still used, changes to that tactic were made over the course of the war.

At the battle of Loos, the British used an adaptation of the wave attack by having three waves, with the third wave being used to flood through after the first two had been pinned down or exhausted.

At later battles, they also used creeping barrages to hammer the area in front of the attacking waves and destroy barbed wire spread across the battlefield.

There have also been instances where the first waves are used to pin down the enemy using suppressing fire, while the the third wave charges the actual enemy line.

While these tactics still amassed heavy casualties, they did not simply charge at the enemy line.

I am not a specialist in this field, so someone who is will probably have a better idea of what was going on.

Losses were greater when countries began to entrench. However, by 1918, many of the problems associated with trench warfare were heavily addressed and alleviated as to avoid high casualties. Improvements consisted mostly of developments in artillery, armored vehicles, and even infantry tactics. Change couldn't happen over night. In fact, many men had to die for nothing for even a shred of relief.

That isn't to say all men had it rough. In fact, activity on the frontline was pretty brief. Some units would be spend an average of two weeks in the frontline trench. The 10th Battalion CEF, for example, averaged a touring time of 6 days. Other units like the Portuguese Expeditionary Corps suffered the longest due to a lack in numbers and the denial of leave requests. I only mention these because there was a little bit of both ineffective leadership and unruly circumstance.

I hope that may start answering your question. Although it seems that improvements were not made, and less lives were lost, there was a lot happening at once and any attempt to implement new strategies in a 4-year-long stalemate was risky. It was really only until the last months of the war, within the Hundred Days Offensive, that mobile war returned to fruition and men weren't simply sent "over the top."

So, all things being equal, commanders did ask themselves, "Why don't we change up our tactics?" But it would take 4 years to answer that question.

EDIT: Structure.