Um... you really think that just because we have avatars of things other than us we are hiding behind something?
I don't like pictures of myself. I don't think I'm particularly ugly or anything, I just don't like pictures of me. So I rarely have them taken, let alone put them on a site. It's as simple as that. I'm the same person online that I am in real life: same interests, same personality, same values. Just because I'd rather have my profile picture be Sokka than myself does not mean that I am trying to hide myself.
Beyond that, there's the whole thing about stalkers on the internet. I'm not paranoid about it, not by a long shot, but that doesn't mean I'm going to go posting my picture around everywhere.