If I take a copy of the Mona Lisa and draw a luxurious moustache on it, I now own the copyright to that moustache-bedecked Italian's image. Sure, the original image is still public domain, and if someone was to crop the moustache out of my version the bit they'd be left over with would be free and clear of my copyright. But if I use an AI to generate an image and then do the same thing to it how would you even know which bit to crop? And what value would there be in the "leftovers"? Might as well just use your own AI to generate what you need.

I think a lot of AI-hating artists feel that if AI-generated art is declared uncopyrightable they'd "win" somehow. I don't think they'll see the results they're expecting, if that comes to pass.