Many reasons, first of all like many people will comment here just to boost post count, hypergamy, tho as much as i know hypergamy is just the result of all the following points?
So first of all we have SM validation, girls pretty friends (ugly bitches always have a pretty friend its inevitable) hype them up by saying shit like „gurlll i wish i had ur lashes“ and these chuzz then think that they are the shit because they have one upped their friend on their biggest flaw. Second, its socially wrong to criticise girls about their looks so they dont really get confronted with it. For boys its normal to get called ugly by their friend and unnormal to get a compliment (which is also bitches fault because they would call u gay but who cares, they are women), whereas girls are sooooo incredibly glazed. Then we have briffaults law which points out how most relationships are abt making the girl or female happy and neglect the man. This is so far ignited into society that former partners of those girls women bitches whatever u wanna call those whores have put the female over themselves, which makes women used to beeing glazed.
I hope this helps u understand those type of humans a bit more (ik i yapped alot im bored rn)