Because it's just a game? People don't want to play games that make them feel bad? Maybe we are all soulless bastards that just like killing random people for fun?
For the last 20 years designers have toyed with the idea of culpability and consequence, of getting players to consider their actions in moral terms. Fallout 3 had its karma system, which rated player actions as good or evil, affecting their reputations with any non-player characters they met. Bioshock famously provided us with a choice of whether to harvest or rescue the little sisters. And Mass Effect judged actions on a dual system which shaped lead character Commander Shepard as a “Paragon” or a “Renegade”. These systems are all flawed of course – morality is not a binary paradigm – but they at least attempted to frame player actions in a way that wasn’t purely systematic.