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.
