The real reason - it'd be pretty damn hard to make the player feel like they were fighting for anything. WWI was started pretty much exclusively by imperialist dick-waving. Then again, retarded, vindicative nationalism played a big role in getting WWII started (mainly in Germany and the like). But you don't get to fight Nazis in WWI.
Not to mention the monotony of the combat.
First and last level: You get mowed down by a German machine gun within 30 seconds and slowly bleed to death in no man's land. The kicker is - your sacrifice is in vain, the battle is a stalemate, and the war drags on for another three years. Oh, and the peace terms directly contribute to another war that kills even MORE people. Bye!
Not to mention the monotony of the combat.
First and last level: You get mowed down by a German machine gun within 30 seconds and slowly bleed to death in no man's land. The kicker is - your sacrifice is in vain, the battle is a stalemate, and the war drags on for another three years. Oh, and the peace terms directly contribute to another war that kills even MORE people. Bye!