
If you really want to know, for me personally it's because most harems are poorly written, self-insert, male-centered fan service stories. It'd be fine if the harem actually added something to the story, but if you take away the harem it's still the same story. Off the top of my head I can only name one story where the women NEEDED to be romantically/sexually involved for the plot and the MC's ploy. Very few exceptions, unfortunately.
It's such a turn off because often the female characters feel like "trophies" to be won. They're introduced as smart, strong, rich, beautiful, but then when meeting the MC their IQ drops and suddenly they're powerless. Or whatever motivates them vanishes and is replaced by the need to be liked by MC (⊙…⊙ ) like bruh... you're the only remaining heir of a kingdom, you have more important things to do than fawn over a man.
The only harem story i've seen done well are palace harem fights. And those are great mainly because they focus on the women and not the guy. Yes, they're fighting for one man but also not. They're fighting for power, for survival,
for revenge, for their families. So much intrigue, so much drama.
I understand why people "hate it blindly." It's not that ALL harem stories are bad, some may be good, yes, but with so many other good stories, who would want to waste their time reading a story with a great start only to be put off halfway? If this happens enough times it's not surprising why harem stories are put on blacklists. We're not born disliking harem stories, you know. The dislike grew over time.
Reverse harems don't get the same treatment simply because they aren't on the same level in terms of quantity and popularity. You don't hear as much complaints because there aren't enough people who are complaining and not enough stories to complain about.
They're also written for different audiences. Because they're written primarily for women, reverse harems focus more on the relationship between MC and the supporting characters. This, in turns, leads to the characters being more developed and three dimensional. Unlike typical harems where the supporting characters are dragged along to embellish MC in glory.
Or maybe like the other commenter said, it's all about preferences. I don't think that's hypocrisy tho. You can like the rain and not want to be wet at the same time.
As long as characters are good harem is fine . But ppl on this site just hate it blindly even though they love reverse harem