Re: Why assume that?

Yes, but this has nothing to do with beauty. Accepting of what one is born with and maintaining your body to be fit and healthy is fine and will produce better results than any amount of fussing over appearance.

Markers of health are extremely correlated with conceptions of physical beauty. More so in the things that we can change, such as body weight / mass, dental care, or hygiene, than in those we cannot, such as skin color.

See above - nothing to do with beauty and appearance.

Beauty standards have varied across time and space, but no culture ever preferred BMIs in neither the rank of dangerous obesity nor the rank of dangerous thinness. Smoking yellowens your teeth, but no culture ever preferred yellowed teeth to whiter ones. And so on.

and this doesn't come from an obsession with how one looks.

I think your definition of obsession is different from what I am used to read.

There are much more important things to devote one's time to than physical appearance.

If by that you mean "you don't need to spend 6 hours choosing a dress" you are right. But I suspect you mean "why wash your feet? - true friends don't care your toenails are fungy"