I've never understood this. I have a few possibilities in mind but can't really know.

And while I can't read men's mind, I've read a lot more women talk badly about their appearance or share how insecure they feel than men. Most men don't even think that much about their appearance it seems.

One of them is women "acting" like they have low self-steem to avoid being critisized by other women as thinking too high on herself and being too cocky.

One of them is women being too competitive about their appearance and competing against the top-10% of women. even when they're not around, because the idea of their existence makes them feel insecure.

One of them is men being delusional and just not caring to the same extent of women because they feel like they're attractive even when they're not.

One of them is women not having to make the first move. Obviously "first mover" has the advantage of using his personality, while the person who is being approached has to rely on her pure appearance unless the guy is an unicorn or something.

One of them is women being more into social media (specially Instagram). In there, anyone can be tricked into being ugly, because you see a lot of women with manekup and filters and you start to compare yourself to them.

One of them is men having more hobbies which don't involve socialization to don't worry very much about such a thing (was real for me when I used to, I've really never even thought about my appearance).

And I'm pretty sure there're some thing I'm missing.