Because you have to be given permission first. You can probably post on other people's threads in the "Introduction" section, but I think that's it.
The way that you're given permission is to create an intro like the one Spammie linked. (That's why she asked if you could tell us about yourself)
I know other forums don't require the approval process...but on other forums you find a lot of...riff raff. This helps with that.