The thing in this case is that AI generation software like Midjourney are free and so easy to use, you can generate more than a thousand images in a day if you really wanted to.
So someone could just hope to get a few bucks by submitting as much AI assets as possible. Which of course means that the marketplace will be flooded with AI assets of low quality.
Which of course absolutely sucks for the artists who sell real work, which they often spend hours or weeks to make, only to be outpriced by someone who basically just pressed a few buttons & changed a few parameters.