From each other, I think. Me, I live on-campus at a reasonably large college, so I see maybe thousands of teenage and twenty-something girls regularly, and I've seen one girl wearing an obvious crop top in the past year. Which I noticed, because it was so unusual. But if you don't see many women in real life, you would have no idea what they wore around, so you'd have to make something up or copy what the guy before you drew. (And the guy before you was copying the guy before him was copying the guy who once looked up from his coffee at Starbucks, saw a girl with a belly shirt and hiked-up thong undies in the line, said "Huh, I guess girls wear that kind of thing," and drew all his female characters wearing it.)
They'll get phased out eventually, I'm sure. Probably to be replaced by something even more dated and silly-looking, but hey, what are we in comics fandom for if not to giggle over the art?