Idiot Threadstarter... Even 20 years ago, there were hot farting models (i.e. Renee)
Doubting it's the case with these models, but there is such a thing as sensitivity/allergy to sunlight. My wife has it, and if she isn't coated in sunscreen during the summer, her exposed body breaks out and she gets tiny bumps on her skin. She has to reapply at least every hour if we're outside for an extended period of time.
They couldn't make it in sexy porn so they stick to fetishes. If a chick is really hot in porn, she will make big money and not be making fart clips. Even the hottest chicks who make little dough are just making softcore clips.
Also, really hot chicks have boring fetishes. I've asked lots of dimes to fart but they usually into tame shit like choking, light S&M, etc. Girls want to be kinky, but true kinky girls are rare. The hottest chicks I've seen make "fart porn" have genuine fart fetishes and do it for fun/free or they're gainers who fart as a side fetish.