Since the real world is full of violence every day, it's the most basic concept any gamer can relate to. You see it on TV, on the streets, on the internet, violence is everywhere.
Sure, you can make games completely without any kind of danger or violence, but that is why its harder to relate to and create immersion in these games. I'm not saying it's impossible, but it's a much needed component. In Portal, you were in danger of falling to your death, get crushed or shot...etc. In Mirror's Edge, the same. Even the old adventure games, where you cannot technically die, had some sense of danger incorporated, just think back a little.
It's a basic concept, that's all.