You are not missing anything: you are asking the key question in that process. We don’t have an answer.
As a society, we agree that you, a human, copying art you’ve seen is original (or at least derivative enough that it warrants a new copyright).
We agree that a photocopy machine, or rather a human operating one, is copying art in a way that is not creative enough to warrant a copyright — so the human goes to jail.
As a society, we have not agreed upon how transformative a “work” from a generative AI has to be to count as original, or even if it can. A recent decision claimed that no machine work without human effort would count — ignoring the effort to find good prompts, pick an image and edit it further. An AI is different because no much was “trained” to make art before AIs: some animals were (elephants famously), and who owns their work is tricky — there’s a case of a photo, a selfie actually, taken by a macaque that is really interesting in that regard.
https://en.wikipedia.org/wiki/Monkey_selfie_copyright_disput...
My recommendation would be to have a similar system to what we have for music (and I think plays) where there are two or three “authors”: the person who wrote the score, optionally the person who wrote the lyrics, and the interpret: the singer. It gets really fun when you have orchestras, animals involved in making music, or sound-sampling like in RnB.
We could have original human authors and AI adaptors, each paid based on use. If I like your work, I could commission OpenAI to make “a portrait in the style of alistairSH” based on your DeviantArt work, but of my mother (whom you’ve never met), using photos that I’ve uploaded of her. I can pay OpenAI, who would pay you. Style vs. theme could be separated with current AI technology, although that could get just as complicated as every intellectual property process very fast.