Why are people so adamant that beauty isn't a set standard, even a little bit?

People always complain about this but there is a reason a healthy just the right weight, curves, and large breasts are attractive. It's because those are all symbols of fertility and health. Sure there are minor preferences like race, size, etc and some wierd fetishes but don't most men like those three things in a woman? Also if its ok for some people to like larger girls or a different race why is it considered bad to not be attracted to most of a particular race.