-They simply aren't into real women/men (so you couldn't even pay a model to do what they're looking for).

-They want to play out fantasies that would be hard to coordinate even if you could pay models (I guess this is more on the video side of things, but a string of photos can put be together into a comic)

-They want to generate imagery that would be illegal

Based on this, I would guess fetish artists (as in illustrators) are more at risk than OF models. However, AI isn't free. Depending on what you're looking for, commissions might be cheaper still for quite a while...