It's not a matter of intelligence or sentience. The key question is whether the output of a prompt is fully predictable by the person who gave the prompt.
The behavior of a paintbrush, mouse, camera, or robot arm is predictable. The output of a prompt is not (at least, not predictable by the person who gave the prompt).
In most experimental work, the artist does make a direct contribution to some key elements of the work, for example framing or background. Which is all that's necessary, you can still obtain copyright over something that is only partially under your control.
If an artist gives up all direct control over an experimental work - such as the infamous monkey selfie - then I think they should no longer be able to copyright it.