Because art, commercially viable art, is 1% inspiration and 99% perspirarion. AI can generate the first inspirational image but cannot do the 99%, all the hard work, to turn that image into a viable product.
I was in the grocery store the other day, needed to use the restroom, and saw an AI-generated picture, framed, hanging on the wall in the restroom stall. It was even "signed" by "Meta AI".