If you're talking about profiles specifically, I think the lack of info is more likely due to the structure of forums. I don't think user profiles are high-traffic areas in many forums since the action is a bit focused on threads. Something like facebook puts the action on your profile, so there's more incentive to load up your page with all the juicy details.
Put another way: I don't think it's secrecy; I think it's a lack of incentive from a UX perspective. Which isn't a bad thing; it's just the nature of the beast: less about individuals' activity/info, more about topical conversation.