It's not that. I can easily believe a good DNN could today generate better (by any kind of objective standard) music than one created by top composers.
But here is where we discover another component of value that exists in arts: the connection to human mind. Given a piece of music you like that you don't know the provenance of, you'll probably value it different after learning it was composed by a human vs. generated by an algorithm on the spot. It's irrational, but that's how things are with people.
A similar case would be of story generation in games. I don't know about you, but for me, learning that a story or a quest line was procedurally generated essentially destroys my suspension of disbelief on the spot.
It's like a defense mechanism. You value the work less because it was computer made rather than to admit to your flawed value assignment and vanity or human supremacism.
It's the difference between a friend giving you a note with words written on it, and finding a bunch of sticks arranged to the words by chance while strolling through the woods. There is nothing "flawed" about that valuing those things differently, how I judge things as valuable is entirely arbitrary, and entirely up to me.
And with AI it's not even by chance, even mediocre results are produced only after processing the input of countless humans. It's infinitely less cool of a coincidence than something that is actually random, like sticks forming a sentence where the wind blew them, and a complete NOP when it comes to human self-expression.
