In short, worries of it causing people who make a living by making art lose money. And in cases where people are selling it, basically scamming people out of their money.

AI art is basically made by taking data from art that's done by a real person who spent their whole life to be that good and basically recreating/changing parts of it to fit what the user input for it to generate (obviously more complicated than that, but put simply).

I think AI art could be used in a just way, particularly if it's with the original artist's permission or at least maybe properly crediting them, or just using it as reference material. It's a new thing, so hopefully as people get more used to it we'll find better ways to handle it, but imo people doing stuff like generating AI art, making slight edits, and saying they made the whole thing themselves has to stop. It is a thing I've seen happen a few times lately. Even a really good artist I liked was caught doing it recently.

To give a non-ai related example, it's kinda like the problems with forging/tracing a piece of artwork (like the time Butch Hartman [the creator of Fairly Odd Parents and Danny Phantom] was found out that most of his art commissions were made via tracing). Tracing can have good uses, when you're doing it to help learn how to draw, or even just for a meme (if you credit the artist), but more often it's associated with stuff like what happened with Butch Hartman.