Whoa there's strong opinions here.
I don't read much in the harem genre, but I think there's a reason why harems are so popular.
Harms are romantic ensembles, much like how the Avengers is an action/adventure ensembles. Readers get to pick their favorites, compare and contrast characters, promote their favorite ships, attack their favorite punching bags, argue about who's "best girl" or "best boi" etc. Authors can create a menagerie of characters that fit all kinds of tropes, each to fit a certain kind of reader. And with so many characters, ensembles make it easy for authors to write new content--all they have to do is an episode with two or more characters in the ensemble doing something, with the others off in a B-plot somewhere.
The problem I think is that the authors that choose to write harems often are often not writing for an audience, but for their own wish fulfilment. There's nothing wrong with a wish-fulfilment story, but those stories are going to have a limited appeal. A story that reovlves around five monster-hotties or anime waifus clamoring over a sanitized author self-insert is not likely to win many prizes.
Relationship stories require brutal honesty from the author and ability for self reflection, and a deep and nuanced understanding of relationships between wildly different personality types order to make that diverse cast of characters work. But it may be that the kinds of authors interested in writing harems are writing precisely because they don't have that nuanced understanding yet. The large cast allows them to switch between character arcs without going into any particular one too deeply--because they just don't know how to yet.
And that's fine--it's all part of an author's growth. But if that's not something that's up your alley, as it's not necessarily mine, I would just read other works in other genres. Let people enjoy their sparkly werewolves and E-cup demon-girlfriend-senpais.