In my opinion this boils down to entrenched patriarchy.
My girlfriend recently told me that I did this to her when we first met. It was so long ago I didn’t even remember. This was long before I read books like Feminism is for Everybody and got a sense for just how awful the day-to-day lived experience of women is compared to men.
Like most guys, once I found out, I fancied myself as above all of that nastiness. It wasn’t until my girlfriend pointed out some things that I did that were, in fact, sexist, that I couldn’t just wave away as misunderstandings, that I had the awareness to dig deep and figure out what was going on.
Things like bigotry, racism, sexism issue forth from deep in the brain. It’s virtually impossible to completely root out, and it’s difficult to get to the bottom of any of it.
The only solution is to constantly be on guard for it. Most guys simply aren’t up to the task, they are unable to be a part of the solution all by themselves. They are like those old grandparents who are racist, been racist their whole lives, but understand on a purely intellectual level that using the n-word is wrong. Just having the understanding isn’t going to erase 60 years of lived experience. They can stop using the n-word, but they can’t actually fix their racism.
Most guys have lived their entire lives in a culture of patriarchy. Asking them to not look down on women is too much for them. You can ask them to stop treating you like crap and point out to them ways in which they treat you that make you feel like crap, but unless they are willing to make it a major life goal, like I have, actually eliminating sexist behavior completely is just too much for them.
Men today are like white people were 50 years ago. Even the best amongst them had mountains of racist lived experience and it just came out from time to time, black people just had to tell themselves, “he’s better than the rest,” and just not make a big deal out of it.