Part of it is the very mechanics of gaming, they're all built on a core of goals, rules, challenge, and interaction. When telling a story, the four basic forms of conflict are man against man, man against nature, man against self, and man against society. Violence is an easy vehicle for three of those conflicts, and especially lends itself to active gameplay loops. Mind you, I'm referring to violence as acting to cause injury, because there are a lot of games that are built around fighting with zero gore or death.

The other thing is that violence is just very popular. If you stop to really consider it, how much entertainment is free of violence? How many shows and movies are completely nonviolent? How many books don't have a single fight? There are genres that typically avoid violence, but even then you'll still find members of the genre that contain physical conflict. Plenty of romance and dramas that are steeped in fighting and death.

At any rate, not that my perspective's any more valuable than anyone else's but I really haven't seen a demand for violence that's lower that the supply.