Copyright is held automatically by the person who produces the work unless otherwise specified in a contract or agreement. I am a commission artist, and I retain all copyright to all of my work, per my terms of service, unless otherwise sold via agreement, and I charge extra for copyright.
Someone can't commission me to paint something, and then turn around and sell prints of it themselves. I still did the work, and they need my approval before they can legally make money off of it.
The reason why AI art is so devisive right now is because thousands of artists have had their work scraped and the AI trained on their work without their permission or compensation, and unlike a human who looks at other art for reference or inspiration, an AI can look at a dataset and pump out thousands of similar images in a very tiny amount of time.
If the AI author or the user generating the image owns copyright, you can kiss artists selling their own work goodbye. The space will be flooded with "AI artists" who create work at a far faster pace than anything a real artist can create, for a fraction of the cost. The people who train AI on this art are profiteering off of the hard work of the people who spent years of their lives developing a skill.
I think you mean prompting. Commissioning an artist in the context you're thinking of implies a written contract, signed by both parties [0]. This is the first reason where it isn't the same, and I believe is enough to consider new laws in the situation.
Even if the new law just codifies the process you're thinking of, it's better to get ahead of the situation, or else we make a mess of things like DMCA, etc.
Thaler tried to argue that he and the AI were in a situation similar to "work for hire" with an artist, but there was no human input to the AI. But, the copyright office holds that non-human entities/events are not granted copyright protection and therefore have no rights to transfer. So, they rejected his claim.
This is only true if it is written in a contract. In general, the artist keeps the copyright. Photography is the same way: Photography studios own the copyright to wedding pictures, school pictures, family portraits, sports functions, and the like. If you pay a photographer to take a family picture, you have to get permission to use that picture for a greeting card unless you've signed a form giving that permission to you.
Similar rules apply if I sell some artwork that wasn't commissioned. Someone else might own the original, but only I can make prints and post a picture where I choose. Unless, that is, we have a contract that says otherwise.
[0] My editorialization. The rationale of what is and isn't a work-for-hire was not some principled decision but just a list of whoever wasn't in the room when the last major copyright law was drafted.
Most holders of copyright are non-human entities like Disney.
The legal system considers groups of humans to be as equally protected as and have the same rights as individual humans.
If an employee makes something at work, the employer is legally the creator of it.
If someone contracts someone else to make something for him, the copyright in the work vests in the hiring party essentially when it's part of a larger work and the contract says it's a work for hire. A one-off commission on its own does vest in the artist.
(Disclaimer: I'm a lawyer, but I'm not your lawyer.)
[1] https://copyright.gov/circs/circ30.pdf [2] https://en.wikipedia.org/wiki/Community_for_Creative_Non-Vio...