Because they're trying to gain admiration in the eyes of people they'll never meet offline?
Perhaps they place such an importance on looks, they do not believe their personality will count for squat in a place where looks don't matter.
Or maybe they're living out a fantasy of looking more like their "ideal", without actually having to put any effort in.
There's a lot of potential reasons, I suppose.
My avatar and I do not look alike... but in everything else we're very, very similar (only I'm real, he's not

). It's like someone reduced me to a handful of core elements and chucked me in a badly written, badly coded, dialoge heavy adventure game! Pity I only got a bit part