For art to be art you need space to express yourself through individual choices:

• play an original song on a real instrument, and you have the entire artistic spectrum to yourself;
• if you make the music for it out of individual pieces, you narrow that range. The sounds are not yours, only their composition and words;
• when you record a cover of a rap song over some elses beat, you further narrow it down to your performance only. Its still artistic expression, but to a much less degree than no original song;

In a prompt generated image, the image itself is not your expression. The prompt is, but compare the amount of choices you need to make with a painting over a prompt, and you’ll see how much less of an art it is.