But you're asking Pixar what the motivation for your characters are?
Stop trolling dude. You know they didn't go to specifically to ask why Marcus n Dom r shooting sh*t. They went to go find better ways to convey emotion, motivation, themes etc thru a company that has that art down in spades.
Like a movie like wall e is all about isolation. And they did it masterfully. Epics biggest problem is its hard to deliver mood in an high octane shooter. Half Life and Bioshock had it in spades(using mood, tempo, setting, enemies to convey themes and story without stopping gameplay to tell you how to feel) but gears isn't in that vain.
Like everyone else, we try yo learn from other pplwho do something well.