Probably because you see a lot of art that looks similar to this art style, doesn't necessarily mean this art is AI. The point I'm making is, this art style existed before AI became a crutch for game developers, and AI needs a source of art or a specific art style to borrow from or at least be a inspiration of sorts to generate any AI art. That's why so many AI games all share this kind of art style in some way, shape or form.