A very naive thought, but isn't it a bit different for porn sites because of the existence of orgasms? If I'm watching TikTok and a really good video comes on I am encouraged to watch more but if there is a really good porn video I'm ... Done? At least for that session. So it's like they want to keep dopamine levels not cranked up, but at sustained medium levels.
Again, I don't know anything about UX, please tell me if I'm wrong.