What was futile about going over the top? The battles of trench warfare were quite bloody, but it must be said that in terms of casualty rates the battles of mobile warfare, in 1914, 1918 and on the eastern front were VASTLY more bloody.

In 1915 the French were able to achieve a break-in of the German lines at the Second Battle of Artois; soldiers of the XXXIII (?) Colonial corps captured Vimy Ridge, and it was only the failure of reserves to reach them that led to their being pushed back. Similarly, at Neuve Chapelle and Loos, British troops were very close to making at least a modest breakthrough.

At the Somme in 1916, the French took all of their objectives on the first day, and for the rest of the battle enjoyed consistent success against the German Army. XIII and XV Corps of the British Fourth Army also took their objectives, and the British were subsequently able to capture important ground, and inflict great losses on their enemies. Similarly, the French held their ground at Verdun, and under Robert Nivelle went on the offensive, taking back the ground they had lost.

At Arras in 1917, the British enjoyed even greater success; More ground was captured than in any other British offensive before, while the Canadian Corps took Vimy Ridge. Messiness Ridge was a stunning success, and even in the Battle of Third Ypres the British could say that they had inflicted great losses on their foe, seized Passchenadele-Staden Ridge, and I'd say come quite close to a breakthrough.

Similarly with the French; the Nivelle Offensive DID in fact lead to the French Army mutinies, but it did succeed in capturing parts of the formidable Chemin Des Dames Ridge. This lay the seeds for a stunning French victory at La Malmaison, which completed the capture of this formidable German position.

As to needless slaughter, I'll leave you this answer that I posted earlier in an ANZAC question (hence the references to Australia and New Zealand):

The losses make me sick to my stomach as well. With hindsight, I can see that mistakes were made and could have been avoided, as could some casualties, but in light of communications difficulties, the fickle nature of new technologies, and the general fog/friction of war, I can understand why mistakes, when they were actually made, were made. In the end, casualties are unavoidable in war; WWI was no exception. It should also be emphasized here that it wasn't easy BEING a commander, either an officer or a general. By the end of 1914, the Austro-Hungarian Officer Corps had suffered 50% losses, and losses among British Staff Officers were so bad that they were ordered NOT to go to the Front (most ignored this). 86 German, 78 British and at least 47 French Generals died for their countries in the Great War, and yet their kind are reviled as cowardly, repugnant human beings today.

However, one must ask oneself why they were fighting. Britain could not standby and allow Germany to exert hegemony over the continent, the High Seas Fleet resting in the Channel ports like a gun pointed at Britain's head. The Dominions relied on Britain for security and economic enterprises, and any threat to Britain was a threat to that empire of which they were a part. This was more pressing for the Aussies and the Kiwis, in light of German possessions in Samoa, the Mariannas and Marshalls, and Kaiser Wilhelm's Land, and the rising power of Japan.

As for the continent, was it not right for Belgium to be able to decide it's future, to remain an independent state and not be partially annexed or made an economic vassal as the Germans intended? How about the French? Was it not right to prevent the Germans from subjugating that country, the home of millions of 'Poilus', fellow soldiers of the French Army?

There's an Australian Historian that I recommend you look into; he died years ago, but his name was Trevor Wilson. He was no defender of the Generals, as his and his colleague Robin Prior's books on the Somme and Passchendaele can attest! In 1986, he published one of the best accounts of the British experience in WWI I've ever read, called 'Myriad Faces of War'. In the final section he asked that, if the First World War cannot apparently be dubbed a 'good war', unlike WWII, can it not at least be considered one of 'Freedom's Battles'? Would the democracies/mostly democracies of Britain, France, Italy and America, even the peoples of Russia, not pay a terrible forfeit in the event of a Central Powers Victory? Would not Democracy in Europe have been greatly curtailed, even more so than it was, by a triumphant Kaiserreich? Was there REALLY so little at stake, as most people assume?

Food for thought! :) I'll leave you with the final passage of his book, which references Field-Marshall Haig's 'Backs to the Wall' despatch, posted below:

http://www.firstworldwar.com/source/backstothewall.htm

"This might be dismissed as the empty rhetoric of an impoverished command. Yet there are grounds for not doing so. For somewhat earlier in the same crisis an identical view was being expressed in a notably different quarter. A passage in the radical periodical the Nation, quoted on a previous occasion, deserves repetition here. Massingham wrote:

In the full brunt of the German assault on France, the true character of the war stands revealed. Vain projects of imperialism obscured it, and vainer diversions of strategy. Both have disappeared... the war emerges from these mists not as a war of adventure, but morally and physically as a war of defence.... The war was not for colonies, Imperial Ambitions, or a balance of power. It was to teach militarism a lesson of restraint.

What seems of particular note is the congruity between Haig's affirmation that the issue at stake was 'the safety of our homes and the freedom of mankind' and Massingham's characterization of the war as a 'war of defence', a war to 'teach militarism a lesson of restraint'. In short, despite their great differences in background and outlook, each was prepared to claim that this was in truth one of freedom's battles.

Perhaps in so perceiving the conflict, the traditionalist Field-Marshall and the radical journalist were both deluded.

Perhaps, on the other hand, they were not."

TL;DR: They didn't rebel (at least not frequently) because they had a job to do (ie win the war) and most believed it was a job that needed to be done.

I'd recommend checking out Leonard V. Smith's Between Mutiny and Obedience: The Case of the French Fifth Infantry Division During World War I. Your question begs a very long multidisciplinary response and Smith's book is definitely the best book on the subject I've read.

Long story short, Smith explains that command authority reflects "a negotiated balance-of-power equation." Some of the things being "balanced" are the background of the troops- French soldiers with a long history of democracy vs. Russian conscript who were only 1 generation removed from serfdom. The strength of the unit's position is another factor to balance- the more secure the position, the stricter officers could enforce discipline because they could count on reinforcement in the face of mutiny. The attitude and ability of officers and NCO's are a factor. Conditions on the front obviously play a role- the greater the starvation, deprivation, danger, and general misery, the more discipline flags.

There's a lot more to it than can be summarized here. But Smith's work is one of the best things I've read on the nature of authority and power, and it just so happens to use a WW1 unit as his case study, so it should be right up your alley.

It may be worth pointing out that several of the major armies engaged did, at some point, suffer something like a mass mutiny, or refusal to attack. After the failure of the Nivelle Offensive, several French divisions refused to attack again; they were willing to hold their trenches but not to attack. The Italian Army more-or-less collapsed after the eleventh Battle of the Isonzo, and when the Austrians attacked with German support, were driven back in a near-rout as far as the Piave, with ten thousand killed and a quarter million (!) taken prisoner. Conversely, when the Italians (with French, British, and US support) counterattacked in the battle of Vittorio Veneto, there were several instances of Austrian troops refusing to obey orders to attack, which contributed to the Italian victory - though Austria-Hungary was by then pretty clearly dissolving. The Czarist army basically demobilised after the Revolution, and famously "voted for peace with their feet". Even the Germans had trouble with looting and indiscipline during their last-gasp Michael offensive, and in their retreat during the final British offensive in the fall of 1918, troops coming in to plug gaps in the line were greeted with cries of "strikebreakers" and "scabs".