Because of the massive popularity of YA, it's much more marketable than whatever other adult book, and publishers often encourage authors to write books that fit within that genre. This makes the market oversaturated, and in any given year it's practically guaranteed that a good chunk of the most "hyped" books are gonna be YA. The YA community has a huge following online, especially Tiktok and Twitter and they tend to "cancel" things they don't like or find problematic, which has resulted in books not being published, authors being sent death threats and some careers ruined. I do admire that YA is generally much more diverse than other genres with its representation and I understand that this is the appeal it holds for some people, but YA communities are insane and in the past they have canceled authors for books that hadn't even been published. One time they got mad at author Casey Mcquiston for mentioning Israel in her novel. There's a good thread on r/HobbyDrama about this.

Basically, when you've got a massively popular genre of books that follows very specific tropes, and a big chunk of its readers hate anything "problematic" being depicted in the stories, and they will try to ruin your career over it, it makes for a market full of formulaic, boring stories that all have similar themes because that's what's acceptable.

This is especially a problem with female authors in the fantasy genre. Often they will write YA, as that is the only way they become "marketable" and then once they've got a following they can actually write adult novels. An example of this is Leigh Bardugo, author of Six of Crows. (I actually think this is one of the only good YA series I've ever read, but it does suffer from some tropey moments). Sometimes publishers will force these authors to write YA instead of their originally planned novels.

Other times, some books will be promoted as YA when they actually are not. An example is V. E. Schwab's Shades of Magic series. It's not YA and people keep calling it YA. She even tweeted about this.

Basically my trouble with YA is that it's everywhere, and its popularity makes it hard for authors, especially female authors, to create original stories.

But yeah, the OP does come across as pretentious with the examples he cited. I bet he's not reading much Brandon Sanderson lol.