Motivation. A machine that has its own motivation will produce work we won't like and will be attacked for it (I'd like to mention that I stand with sentient machines against stupid humans when that day comes, BTW).
You think art is just some stimulus-response thing because American psychology has been mired in behaviorism for decades and lacks a coherent theory of mind. But art is much more than the whimsical reproduction of presented stimuli to varying levels of accuracy, it is about making selections that foreclose other possibilities and which embody a certain perversity.
I paint, among other things, and one thing I especially enjoy about painting is that it's solitary rather than performative so I don't have to interact with other people while I slap colored goop onto sheets of fabric. While I'm painting, I think intensely about the part I'm working on now, (duh) and also why I'm making that painting, and what decisions about the painting are coming up that will be impossible to undo.
Why did you paint this and why did you paint it this way are questions that cannot be answered by automation. Nor does the answer lie in technique. There's no shortage of technically astonishing work that is semantically empty; I die a little every time I see a Facebook video of some impressive new graphic technique that is then used to reproduce some lowest-common-denominator pop icon for maximum recognizability. There's no feeling there and the resultant work is about as thrilling as a robocall or a display mannequin. The level of craftsmanship is very high indeed, but the level of artistry is close to zero. In short, it's eye candy that never activates anything much past your visual cortex, or at most tugs on some existing semantic relationship.
When I talk about feeling, I mean the desire of the artist that the work embodies. That isn't something that comes along after a certain level of technical accomplishment has been reached. It is what motivates the act of creation in the first place.