I work a lot with human AI communication, here's my take.

The issue is our judgement (think value function) on what's good. It's less to do with what the AI can actually do, but more with how it is being judged by people.

Random blotches of colors shapes in an interesting way on a canvas is modern art. It's non intrusive and fun to look at. A painting with less than perfect details such as having goblin hands with 6 fingers (as they often do in AI generated arts) isnt a big deal, as long as the overall painting is cool looking.

A music phrase with 1 wrong note, one missed tempo, one sound out of the groove would sound absolutely garbage. We expect music to uphold this high quality all the way through, all 5 minutes. No 'mistakes' are allowed. So any details the AI gets 'wrong' will be particularly jarring. You can mitigate some of the low level errors by forcing AI to produce music within a DSL such as MIDI, but the overall issue of cohesion will be there.

Overall, generative AI lacks control or finesse over the details, lacks logical cohesion. These aren't problems for paintings as much as music.