There clearly is "a" market because I have seen mentions of a fic being commissioned on both Archive of Our Own and kemono without even looking for such myself.
However this market does seem to be fairly small and/or hidden. Even including piracy it's not as easy for a potential customer to judge the author's skill as reading takes more time and effort than viewing images or video, nor is there a decent equivalent to a "sketch" to iterate on without the risk of handing over most of the work for free. Writers can't share censored/low-res versions on social media either like people do with drawn art which is a major factor in publicity and advertising.
And then there's the fundamental issue with "fan" fiction as opposed to the largely "original" (in terms of licences/IP rights not creativity) porn games. Pinup artists seem to be getting away with it, even with Pokémon and other Nintendo characters, but there's always the risk of the rights holders cracking down on you. Combine that with purely text-based media already being less popular than visuals and many amateurs writing out of passion and you end up with fairly little space for commercial success. Toss AI on top of that and the outlook for the near future isn't much better, unless you're proficient in training and operating neural networks I suppose.