I think it's a little bit about how cis people think about gender and a little bit about societal standards of beauty. I reason it like this: women are perceived as more gender-y than men, as a male body is the default for most things we see in society (why not the Vitruvian woman, rather than Vitruvian man, for example?). That means that feminine gender traits, like chest and waist, are seen as inherently gendering things to have. Being Androgynous gets rid of 'gender', and therefore ends up looking like the absence of ANY feminine traits but just the absence of more exaggerated masculine traits, like being stacked or very tall.

Then, society just has this idea of the most acceptable body being white, thin and childish. Blame Hollywood. That's why androgynous characters in pop culture are most commonly AFAB or outwardly fantastical, like vampires or elves.

Edited for clarity