Here's what the US copyright office says about when AI art is or isn't copyrightable:

In the case of works containing AI-generated material, the Office will consider whether the AI contributions are the result of “mechanical reproduction” or instead of an author’s “own original mental conception, to which [the author] gave visible form.” 24 The answer will depend on the circumstances, particularly how the AI tool operates and how it was used to create the final work.25 This is necessarily a case-by-case inquiry.

So if an image looks like AI and you decide to just take it, legally that could be a risky proposition if you don't know the artist's workflow and the situation doesn't exactly match up with settled case law. Afaik most of the market is for custom images, so in practice most of the time it's not going to be a situation of just putting in a prompt and handing over the result but rather a multi step process and a hybrid of different techniques, which could weigh more towards generated content or more towards traditional drawing or image manipulation. The reason to pay someone for that instead of just using AI yourself would be the same as the reason for paying for non-AI art; they have the skills to get better results than you easily can on your own. The reason an artist might use AI is that it improves quality and/or reduces the amount of work.