I've been particularly interested in World War One history for a couple decades now. That interest has driven me to actually seek out the few movies that actually cover WWI.
There really are a dearth of WWI movies especially after WWII. Even movies that do deal with WWI generally use the war as a backdrop - Legends of the Fall; or, they're built around the futility of it all without really dealing with the combat - Paths of Glory.
All Quiet on the Western Front, Gallipoli, War Horse, and 1917 are rare entities.
The reality is that, if you want to know why WWI isn't really covered in cinema, you should just try to write a historically accurate script about the war.
The first thing that WWI lacks that WWII has in regard to cinema is a clear bad guy. The Germans weren't the Nazis in WWI.
What's more though is that it's true that WWI was just a depressing exercise in futility. It was estimated that there were times during the first day of the Battle of the Somme that three British soldiers were being hit by a bullet every second. When Gallipoli was showing full frontal assaults with men being lucky to make it a matter of yards out of the trench, that's what most of the war actually looked like.
There's no way to accurately show what WWI was like and make it a John Wayne, gung ho war movie. That's also why I think that WWI should get more attention. There's nothing fun or glamorous about war and WWI really showed us that as clearly as possible.
Shakespeare wrote in act 4 scene 4 of Hamlet: "...to my shame I witness the imminent death of twenty thousand men; that for a fantasy and trick of fame go to their graves like beds. Fight for a plot in which the numbers cannot try the cause which is not tomb enough and continent to hide the slain."
Twenty thousand men died, just on the British side, during the first day of battle on the Somme. There was not tomb enough not continent to hide the slain. They made no headway and the battle lasted for nearly a year after.
That's what WWI was.