Because I'm happy that she understands that it is just a game, and that it isn't something she finds upsetting. There's nothing particularly challenging about the content beyond the violence, and she understands that it is pretend and is there to be experienced within the game, not taken out of that context.
She's always supervised - she only ever plays when I'm in the room - and her time on it is limited, and at the first sign of her getting upset, or even frustrated and wound-up (as she occasionally is with other games she plays at times) she would be told to close it down and come away.
If I thought for a moment that it was upsetting her, or that she didn't understand that the violence and fright was pretend - or that it was affecting her in any way - she would be off there like a shot. As it is, I'm happy that she has a limited exposure to it with me to be there to help educate her as to the content and make sure she appreciates it for what it is.
Sorry for the long post, but I thought it would help to clarify my position