Because REALISM SUCKS.
Just because something is more realistic doesn't make it fun. I mean, why not make a war game where your main charecter loses his legs, goes home, and kills himnself? Sure, it's realistic, but does that sound like a fun game to you? I think games should stay fake. Borderlands was the first FPS I've liked in a while because it went for fun, not utter realism. In that game headshot isn't always an insta-kill which can be frustrating, but funny. If I wanted to play an overly realistic war game I would join the military.
Also, I am sorry if I offended anyone with my point about a soldier losing his legs and killing himself. I did not mean to come off as a dick by saying that. If I offended you in any way I sincerly apologize.