Mostly because nature presents little intent. This is also why fractals are not considered art.

If an AI is able to present intent clearly and consistently (uhh, intentionally) then it might be able to produce art.

So far, I haven't seen any purely AI work that meets this criteria. They can have style but lack the "story" quality of the art built on shared language, culture. Essentially grammar and semantics of given art form. Problem is similar to though harder than making AI really understand language.

These system generally end at style which is like an ensemble average of syntax.

I don't think we necessarily require intent to consider something as art.

If a pianist playfully improvises, mindlessly, without intent, does that mean she is not creating art? Sure, you can say that her intent is to maximize her own pleasure.. but then, how it is different from evolution?

It reminds me, is intent the same thing as objective? https://www.youtube.com/watch?v=dXQPL9GooyI

The pianist still improvises within the grammar and syntax of the art form, say jazz or pop. (Our devises an entirely new vocabulary and grammar of their own.) This is different from just style add these kinds of improvisation have a set of rules and are very different from form free play.