I'd say it is a mix of wanting anonymity and fear of what might be said. I would wager that most members here retain a fair degree of what could be called vanity. Posting pictures of our clothing and receiving scathing reviews is one thing, but that can all be changed, tailored, and fixed. Posting a picture including one's face and hearing something untoward could be quite damaging.
That said, I've endured years of my family lovingly saying I have a face made for radio, so if I ever post a picture of myself I suppose I'll include my face in the shot.