I think it's probably a learned reaction. There are so many fantastically beautiful men and women out there who are automatically given everything they want because of how they look. People treat them like royalty, and so they simply don't make the effort to become more than just a pretty face. And so the general public has a mental shift into thinking that if someone is attractive, they probably aren't smart.

And because people are attracted to symmetry - not just im people, but in everyday life - they will go with the more symmetrically-pleasing option. That's why beauty is used in advertising: because the brain responds.