PC is meant to cater to a younger audience foremost; it's based on a series of games and cartoons that are mostly aimed at a young audience. You also have a lot of warnings against posting personal or revealing information about yourself on the Internet at large (and I think on here too) and this is something Steve and the staff have to be especially aware of on the forums, I think. The young audience lends itself to more privacy with members and their personal information and it sort of spreads to everyone else, regardless of age.
Beyond that, some people have a facade/image they present of themselves on the forums and meetups/Facebook/personal info could change how they are perceived by people, so some people like the privacy/anonymity of a forum to present themselves as they want to be seen. :P