I guess because people watch movies and TV for escapism. But, it's not only beautiful people who get cast for a show. They may also need people who aren't at all attractive to play other parts. Obviously- they need to be able to act though!
Maybe it's a chicken and egg scenario- maybe the less attractive people out there who would like to act don't want to spend thousands and train for years to get into acting only to find they get no work. The Arts in general contains lots of high risk jobs. Plenty of people likely don't want to risk trying to live like that.
So- the likelihood that Hollywood and the like will go for looks may put less attractive people off trying to train and go for those jobs to begin with.