All art exists because the artist remixes and reinterprets things they've seen before. Everyone does this, whether they realize it or not. You can only make a drawing of a tiger because you've seen a tiger before, whether in-person, from someone else's description, or from someone else's photo or video. But how you draw the tiger is going to be totally different than how any other human could possibly draw a tiger, and THAT'S what makes it special and unique.
Plagiarism is Bad because it's creating Contentβ’οΈ for the sake of creating content, usually for money or clout, without actually caring about the artistic or academic merit of the work. When you create something, there is intention behind it, whether it be "I want to find the link between institutional racism and lowered life expectancy so we can make the world a better place" or "I think these characters should kiss because it would be funny." You do it because you love the thing, or you want to inspire people to do something, or tell a cool story. Yes, you might be doing this for a paycheck, too, but there's that human intention and the seeking of human connection with the reader (or viewer or listener or watcher or whoever).
But when you plagiarize, you're creating content so that there is content to profit from. You have to write this essay for class but you don't want to, so you're gonna make an essay appear and not bother to learn something. You want to put a meme on your Instagram to get more followers so you can attract sponsors. You want to make your story better to get more readers, so let's lift this fight scene that you saw in this one fanfic you really liked, because what matters is a good product, not a good piece of art.
And this is what AI "art" does. It's stealing from artists just to make a piece of content for someone to profit from. It doesn't care about everything being cohesive or having a deeper meaning or the story being fun. Here is a big-boobed anime woman with soft lighting, LOOK AT IT! GIVE ME LIKES! Not to make that connection between art and the viewer, which has always been the reason for art.
That seeking of a human connection is what makes art human, and how that connection is formed and what's within it is different for everyone, and that's what makes something special. But when you plagiarize, you don't give a shit about any of that. You're just making a thing for the sake of a thing existing and you don't care who you hurt in your search for profit or fame.