If beauty means healthier genes, why are some beautiful people born severely disabled?

"Beauty" is a relative term. The idea of what's attractive or beautiful varies per culture. Just like other animals do, humans choose a mate depending upon many different external traits. We can't tell if someone is healthy internally, but if they have what we find attractive externally then we can assume they may be a good mate. Our reasons for choosing a mate seem to be much more complex than other animals, but female birds choose a mate based on his feathers, other female animals choose a mate based on the males' size - so we are similar in that way.