Because many, many people, myself included, would be horribly offended if you could build gas chambers and ovens, round up all the poor, and kill them. There is also, along with the "video games don't cause violence" literature some literature on morality in video games. In fact, there are entire games that are built around forcing the player to make difficult emotional decisions. So it is perfectly understandable that people might not want to play a game where the best / only / easiest way to win is to commit genocide. My point is that the game I think you are describing probably wouldn't sell very well. GTA does well because horrific violence is what the franchise is known for, it is what people expect when they buy the game (GTA also doesn't feature a way to literally commit genocide either, AFAIK).
