It's because generally those groups of people who have historically been glorified over others, in command, socialized that they are superior just for having been born the way they were, feel very threatened and weakened by those privileges(including the very subtle stuff that we don't realize have an effect on the psyche) being taken away, by no longer being in a position where they were one step ahead and above other categories of people from birth.

This isn't just something pertaining to men vs women either, but also to white people in multicultural societies for ex.

Sexism and patriarchy still prevails, albeit on very subtle levels in some parts, so most boys in western countries grow up in societies that instill and condition that expectation of their own innate superiority over at least one category, women. But since in many ways these societies have made progress and are striving towards riding themselves of these beliefs, these boys then have that conditioning tested in the real world and they ofc don't realize they've just been conditioned, it's just the way things are, people don't often question, deeply question, why and how they perceive themselves to be, their place in the world, etc.

So you have men who have been conditioned to a bigger or smaller degree that being a man means you are somehow better than women and that in an interaction with a woman, more so if it's a competition(for a job, a place in university, even a game) you're already in advantage because you're better equipped from the start and if you lose then that means you were impotent not that the woman was just better.

It's kinda like some rich kid who grows up super privileged and is conditioned to see himself as better and having more value than others just cause he has money and stuff. He expects preferential treatment because of that and when he doesn't get it or he ends up without all that money or fails despite having the money he feels like he lost the source of his superiority, that advantage that was there no matter what he did.

Same with men. When they are put in a situation where even that one upper hand they subconsciously thought they had over someone just for being a man, is gone, then it makes them feel threatened to the very core of their identity, like an inner crisis of who they thought they were.

Men often, subconsciously or consciously, don't perceive women as true competition so when a woman challenges that by being obviously smarter, more successful, stronger, etc, they feel threatened to their core or they feel he's being discriminated against cause a women can't be a real challenge to them unless they are artificially given an upper hand.

There was a study that concluded men want intelligent women and are even turned on by the idea of a woman as intelligent/successful or more so than himself, but only when they're not actually faced with her. Ofc not all men fall into this, plenty of men don't feel threatened by women like that, but on average the overwhelming majority do, to some extent or another. So when it came to actually choosing to date someone more intelligent than them, they usually didn't feel as attracted.

These tendencies are particularly high amongst men who have lower incomes and who generally come from environments and social standings more likely to be faced with economical and social harships, who feel trapped and powerless and isolated already, etc. That along with less education and lack of access to a broader knowledge and sense of the world they live in, makes men cling harder to that superiority of their manhood or white ethnicity for ex, because it is literally the only thing they have, they are so disenfrenchised and feel so powerless, living their lives in a world that is narrow and empty. If that subconscious feeling of worth and control they get from perceiving themselves as better than women and ethnic minorities no matter how powerless and incapable and at the bottom of society they are otherwise, is taken away, they're left with nothing.

It's not just lack of education that makes rural, poorer areas more sexist and racist on average. It's this too, this conditioned sense of superiority that is weaved into the narrow definition of this one gender and "confirmed" by centuries of oppression of the other genders, the other races, other religions, etc.

So ofc men feel threatened and, again, the ones who have that superiority, the ones in power, they never want to give that up, they'll always resist doing it, they'll always try and find a way to give a false sense of equality and agency and privilege that actually keeps them in power. In this case of course it's not something that happens consciously but it's not easy to give up that sense of superiority, one you might not even realize you have.

I will give an example. Nowadays I think in western societies there's two major ways in which women are still kept subtly in place. One would be beauty standards and the other being a brand of feminism that promotes itself as being radical and inclusive but is actually not. This radical feminism is the type that comes up with ideas like celebrating a woman using her body to get what she wants from a man. That's not feminism and men rarely if ever do that. Or the one that glorifies using the insane beauty standards and objectification of women to get ahead and get comfortable. Again, no, those are at the core sexist born things. Then, these beauty standards and expectations take up a lot of women's time, money, energy and thinking power and automatically makes them prioritize they looks and put more value in it than in other things. That keeps them objectified and used to it and also takes up valuable time cause imagine if more women weren't burdened by every physical imperfection and every extra pound and having to look all fresh and well dressed and cute at every step.

People talk a lot about the oppression of women through clothing in muslim countries but fashion everywhere is made for the male gaze, for male pleasure. In some cases they are hidden through clothes for male comfort and in others they're encouraged to wear as little and as tight clothing as possible and make themselves look perfect also for male gaze.

Women's sexuality is much more free right now but it's still policed subtly and shamed through all these other stuff. Now you're supposed to be sexy and sexual but only for a select few because no one wants an over used object and if you teach women it's truly fine to be as sexual as they want, they might not want you, they might speak out more when they don't really like how the man is during sex, they might leave men faster and they might actually go pursue the men they want instead more often instead of waiting to be approached. That last part would be incredibly empowering.