The answer comes more easily when you drop the normal framing ("they already have equal rights -what are they mad about?) which ironically is the feminist framing.
This is the same reason you have people saying "Third wave feminism is the bad type of feminism, they're not true feminists like in the 20's, who were simply fighting for equal rights". People who say this have already accepted the feminist paradigm as fact and haven't broken with it at all.
With that being said, are most women even that angry? Yes, you look on social media and maybe you see feminist things being shared but that's just part of the package of the normal liberalism we see today and women are just regurgitating that I guess. Like, when women on twitter post "kill all men", it doesn't even feel like there's much anger or conviction behind it most of the time, like as if they just got dumped by a high-status male or an ugly guy bothered her. There seems to be a lot more anger with people on here saying they hate women (well, sometimes, it's become more of a reflex phrase here now).