I think devs believe it ups the escapism of the experience. The more realistic the warzone, the more it feels to a player like a warzone. Know what I mean? As for gray, sepia-tones, and space marines? It seems more "hardcore" to players, so they eat that shit up. That's why; makes your game more appealing to some people.
Am I one of those people? Hell no. I'd rather play a game about time-traveling lizards in robot-suits fighting a legion of bunny-human hybrids with fire whips than shoot the same Nazis in the face fifty-thousand times.
Unless I'm using an acid-bullet sniper rifle and the Nazis are replaced by Jason Vorhees, his clones, and pet wide-mouth dog-things.[footnote]Borderlands is awesome.[/footnote]