WWI on the Western Front was centered around trench warfare. We endlessly harp on about this but the war extended very far outside of the scope of Northern France. Out of the total 37,000,000 casualties of the war if we look only at the major Western Front battles, from the Marne in September 1914 to the Hundred Days in 1918, they would contribute "only" about 7.5 million casualties. If you look at total casualties from both sides on the Western Front it's "only" about a third, 13,000,000, of the total count.

So we can't discard the fact that the Western Front was an incredibly important front but we also can't pretend it speaks for the entire experience. You look at Russia and they have more casualties than the Western front; 15,000,000 approximately. This goes on further if you look at Serbia, Austria-Hungary, Palestine, Libya, Egypt, Mesopotamia, Tanzania, Kenya/Uganda/Zambia, South Africa, and Namibia. None of these had distinctly trench dominated warfare and, in fact, many had very little to do with trenches and very much to do with very open, fluid fighting with cavalry and all playing an enormous role.

We also have to deal with the fact that even confined to the Western Front "Trench Warfare" is not this ubiquitous "thing" that "happened" for 4 static years. Really the "trench warfare" you're thinking about happened only in 1915 and 1916. I actually just wrote a reply on this so I'll copy paste it:

In 1914 there was a very fluid forms of fighting. Trenches were used sort of but infantry was very loosely dispersed and tactically flexible. Basically men would approach ~70 men wide with about 10 meters between each man and in two ranks separated by about 20 yards. They would skirmish their way up while close range artillery supported their actions. I would not call this "human wave" whatsoever and it's not anything close to it I think everyone would agree.

As we move into what is traditionally "trench warfare", 1915 and 1916, this is especially not true as infantry were not designed to assault trenches, they were designed to occupy trenches. Gas attacks and massive artillery barrages were meant to demoralize, destroy, etc. enemy positions and the infantry to approach with little resistance to occupy what remains of their position. That's not human wave that's cleaning up. However the innovations of, say, the flamethrower being widely used along with the Lewis Gun (portable light machine gun) and a wider application of grenades completely contradicts the fundamental definition of what a human wave assault is. Human wave assaults, are, again, results of having no other tactical options available due to poorly trained troops or no materials or whatever. Even in '15 and '16 it was a combination of gas and artillery attacks leading the charge where the infantry would be using a combination of significant amounts of grenades, flamethrowers, personal 'melee' weapons like cudgels and knives, light machine guns, and rifles. This is the absolute closest it ever got to "human waves" and it's not even close.

From here on out it just gets even less "human wavey". In February 1917 the British published a tactical manual for how infantry was to conduct itself during the assault. In 1916 every major power began to develop their own Platoon (60) and Section (15) level tactics; the Russians first applied it in the Brusilov Offensive and the British were the first to apply it to their entire army. A screen of riflemen and bombers (Grenadiers) would approach the enemy trench and begin bombarding it with, well, grenades while riflemen covered the cutters who got the remaining barbed wire. Meanwhile the second line of the platoon would be comprised of rifle grenadiers (yes they could shoot grenades out of rifles at this time), mortars, and the Lewis Gunner(s) would provide immediate support for the assault. The Lewis Gun in particular was seen as the 'howitzer' of the Section and Platoon (each Section had 1 Lewis Gunner out of 15 men). Once the enemy trench was cleared with grenades, the Lewis Gunner and Riflemen had sufficiently pinned down any other resistance, the men would rush into the trench and begin clearing it with close combat weapons like clubs and knives and pistols and shotguns.

Now I know what you're thinking because it's what I thought when I first started reading this kind of information; holy crap this is 1917 in WWI and this looks like a distinctly modern way of waging war. That's because it was. If you look at an assault on a fortified position in 1944 the infantry would be acting almost precisely the same they were throughout 1917 and 1918. From 1918 these types of concepts would bleed upward into the Operational and Strategic levels where the Allies got much, much, much better at applying front-wide pressure but by that point the tactical way of fighting had been sealed in.

Ultimately trench warfare happened because both sides wanted it to happen along with a bunch of natural factors. Southern France was not conducive to offensives due to heavy forestry and mountain ranges; all that was really available was from Verdun to the Channel; basically the shared border with Belgium. So a very restricted front with two massive armies. You also got the Germans who fundamentally do not want to engage the West any longer; the Schlieffen Plan fell apart and they needed to hold off the West while they dealt with the Russian juggernaut. So the Germans chose the most defensible positions in France, dug the hell in, and basically held their ground to bleed the French in particular dry. The Allies were willing to play ball because they could out-attrit Germany but, ultimately, also had to maintain offensive posture for a variety of reasons. As time went on they got much, much, much better at doing this.