The only interesting games made about WWI involve air combat. A realistic ground combat game set during WWI would just be horribly depressing. A more pointless waste of human lives, the world has not seen.
I think people like WWII as a setting for video games because it has such clear heroes and villains. That kind of narrative simplicity is lacking in most wars. It's hard to find a greater real-world evil than Hitler and Nazi Germany. Likewise, WWI didn't have any real "winner," a bunch of people just died for no good reason. Thousands perished fighting for months over a mile-wide strip of land. Stuff like that isn't engaging or fun, it's just awful.