It's mostly because a lot of the professional artists charge upwards of thousands of dollars plus a "commercial use" fee. While I get that artists have to make a living, one has to also ask if the work one does is necessary or if it's luxury. If I were a large company with thousands of dollars to have an artist like that work for me then sure, because the going rate is 50-70 dollars an hour for mid-tier professionals, and when we talk mid-tier professionals we're already talking about artists that do near realistic drawings.
The highest end artists can charge whatever they want. It's pretty insane, especially as work gets more and more detailed.
I don't begrudge artists for making all the money they can get, but at the same time I also have to think of what's worth it. Since I don't technically want to put a title character on the front of the story I'm going to publish, I don't necessarily need an artist to draw it, but if I do want to do that then I'll probably want a highly detailed artist that can really accentuate the character on the front.
I guess it depends on how specific I want it to be, but honestly when it comes to art, spending more money doesn't always equate to getting greater results. Since you have to ask for a piece that goes with what the artist specializes in.
AI art is just making it so people who don't need something specific can get high quality looking art, perhaps with no soul, on the cheap. Like AI novels however, it's usually very generic. All the large breasted AI women on instagram are quite generic as well.