Think of that this way.

In a movie, a director asks actors and camera people to move around in a way that captures the vision he is in his mind.

When someone commissions artwork, they are doing the same thing as a director - they want a certain character a certain way in a certain style.

So rather than hire an artist, you would hire and manipulate an AI. For an artist to compete, they would need to be able to better capture a person's vision than a computer and also do so at a rate that's economical, which as tech progresses will be difficult. Already, art that is purely descriptive (I want you to paint me a girl with a sword) is essentially obsolete for human artists to compete with.