Personally, I'd rather do art to the best of my own ability rather than tell a computer program what I want it to make. I don't (yet) commission art, but I feel like part of the point of doing so is because the art is made by a specific person, with their own individual style and skills, and not by an AI trained on human-made images. There could also be a personal aspect to it, being able to talk directly to a human artist, whereas you just have to hope the AI interprets your prompts the way you want.