Shitty magazines like cosmo and teen people for one. All those weird fashion blogs and celebrity media bullshit. Plus all the ethically dubious ways fashion brands market their products to get people to buy more clothes.

Then it's reinforced by their peers and sometimes family.

Sadly marketing and advertising has steered cultural norms for way longer than it should.

The mothers... Then other girls.

Nature is competition, humans aren't immune.(Or nearly as evolved as we think we are). Women historically haven't been allowed physical competition so they found other ways to decide hierarchy.