Um, I sit in the car all the time without a seatbelt. Why would I put it on if I'm parked?

Well, as far as a roll cage in a 911, that's true, because it's not a full cage. Porsche just doesn't want to wrap it for some reason. Hell, they could pad it and cover it in leather or something if they really cared.

I'd say they don't do it because they think it's not worth the time.