Actually I think its exactly the reason why Brad Pitt doesn't get kudos for acting, call it sexist if you want, but it is what it is. A big part of the reason why Depp did all those weird roles initially in his career was to escape the "pretty boy" role that was quickly consuming his acting ability, ironically, that's the same thing with Heath Ledger, most goodlooking actors think their credibility will be shot as soon as they're typecast, so alot of them do crazy roles in order to distinguish themselves from that. I mean, hate Tom Cruise all you want now, his earlier career had some good acting, but it wasn't until Born on the 4th of July, where he's not just the "pretty guy", that for a time, people took him serious.
It's the same thing with any actor that's typecast, if they're worth a grain a salt, they'll try to go against that. It's the same reason Halle Berry did Monster's Ball when Angela Bassett wouldn't. Angela was already established as a great actress, Halle Berry needed something drastic to try and show people that she really could act. Same thing with Charlize Theron in Monster, or Denzel playing the "bad guy" in Training Day instead of the upstanding noble man he always played, but never got any Oscar for. Going against type is almost always what "pretty" actors, male or female do when they feel their ability isn't being fulfilled, and it's also why the Academy seems to award performances like that. This is why I think Heath has a genuine shot at winning one...