Even if you can't fully predict how a work will turn out, you still have control over your artistic processes in a way that the AI user is lacking. Even AI engineers often struggle to figure out what makes their models make the decisions that they do.

But don't forget that this is a question that exists in both philosophical and practical aspects. Philosophically "what is art" is a very nebulous thing to pin down. Practically, if AI users are allowed to copyright their output, they can use it for "plagiarism laundering" so to speak, by ripping off artists' entire collections, training AI on it, and then selling works that are clearly based on those other artists' even if non-identical. This is not something current copyright accounts for, but current copyright was made for a world with printing presses and photocopiers, not one with AI.