Tumblr - Because villains can't have tragic past, wounds from battles or accidents.

Still, though. How many burn victims have been compared to Freddy Kreuger? Not saying this is Hollywood's fault. Just saying people can be insensitive, no matter how many disfigured good guys are out there. And as for positive role models with disfigurements, while it may give disfigured people someone to look up to, when it comes to jokes about "You look like x", that's barely any better.
Say you're wheelchair-ridden (like permanently) and there's this one guy at work or school who only speaks to you in Stephen Hawking impressions. Stephen Hawking is an accomplished and respected member of the scientific community, but that doesn't mean it doesn't hurt to know someone sees you as a one-note joke in their life.