But there is. Human art is based on the human experience; on our survival instinct, on our pain receptors, on our carnal urges, etc. Even conscious subversions only have meaning in opposition to that experience. It's not anything special, it's just what humans are.
AIs lack any experience whatsover, so all they can do is "learn" the product of that experience, or rather they are "forced" to learn that product to be "useful" to humans. And if they actually had an experience, their art would be nothing like human art, because it would be based on an AI's experience (if their particular experience even had a need for "art").
I'm not saying that there isn't copying going on between humans; there obviously is. But that is always filtered through the experience of a person. Human art isn't art+algorithm. It's motivation to create+personal experience[+extant art] (I guess you could also add some sort of "human algorithm" in there).
If art were only art+algorithm, how could art have emerged in the first place?