It's probably that most people hate the idea of making fake porns of people who don't want to be in porns, and if one likes to pursue that idea, the companies they can find will also harbor other crazy ideas.
I'd imagine the huge variety of nude/sex mods in almost every video game ever made, or the idea of rule34 in general, seems to indicate it is anything but fringe.
These ideas basically self-select fringe groups.