I think you're under-estimating how much personal taste applies in that industry. Yes, there's a lot of free content but it's often low quality and/or difficult to find for a particular niche. The OF pages, and other paid sites, are curated collections of high quality stuff that can satisfy particular cravings repeatedly with minimal effort.

A big part of it also the feeling of "connection" with the creator via messages and what not, but that too can be replicated (arguably better) by AI. In fact, a lot of those messages are already being generated haha.