I think its due to gamers being most at home in situations that most non gamer people might cause other people to be afraid or anxious. Most games have some or other negative event at its core; nuclear war, zombie-outbreak, terrorist, alien invasion, a turtle dragon kidnapping your girlfriend.... since we are bombarded with this in our gaming hours our subconscious no-longer perceive it as "bad" or fear inducing, Instead we like it. Most of my nightmares are of rather ordinary occurrences; being at my old job having not captured invoices for six months, the current example I'm working on not working out no matter what I do.
On the positive side now we know how to prevent PTSD... play games.