It's a
simple phenomenon, really.
Hollywood is full of
aspiring actors and
actresses, many of whom wish to become
celebrities. For the sake of this discussion, I will define
celebrity in the way that you think of it (although not consciously) if you're an
avid reader of People Magazine--someone whose
full time job it is to look and be
beautiful. I've decided not to
rail at
Hollywood and
celebrities for
vanity or
narcissism--I've simply accepted that that is their
job, what we pay them for, and they're merely attempting to do this job as best as they can.
So, you've got a whole bunch of actors and actresses whose main intent is to look beautiful which is fine until a casting director needs someone ugly. Now, true, there are a handful of character actors who have made a career out of being ugly--but they're few and far between. So what we get instead of that are the actors and actresses who are merely ordinary, or even attractive, instead of the drop dead gorgeous ones, to play ugly characters. This is especially true in those Cinderella story or Pygmalion type of movies where ugly girl turns into beautiful woman--usually by letting hair down, applying makeup and removing glasses (and perhaps stuffing a bra too.)
Again, we have to blame our culture's need for our stars to look beautiful at whatever cost. Even people like (my favorite example) Vincent Schiavelli, although surely not beautiful in the traditional sense, are perfectly normal people in attractiveness--but have to play ugly people because they're about as ugly as it gets.
This will probably never change. Why should it? Glorification of beauty is what Hollywood is all about.