It's a well-understood self-contained use-case without many externalities and simple business models.
What more, with porn, the medium is the product probably more than the content. Having it on home-media in the 80s was the selling point. Getting it over the 1-900 phone lines or accessing it over the internet ... these were arguably the actual product. It might have been a driver of early smart phone adoption as well. Adult content is about an 80% consumption on handheld devices while the internet writ large is about 60%.
Private tunable multi-media interaction on-demand is the product here.
Also it's a unique offer. Role playing prohibited sexual acts can be done arguably victim free.
There's a good fiction story there... "I thought I was talking to AI"