In traditional publishing publishers view the cover as a marketing tool - Arthur C Clark had negligible influence over what appeared on the covers of his books in his career when 2001 was released. That's not self-expression by any definition. You could hire a human to do art for you if you like, that's also not self-expression, nor is using an AI to do art for you. There's no meaningful self-expression in the act of having other people make things for you, they're doing the expression, you're enjoying (or not enjoying) the result.