Slop.

a long time ago i was reading a discussion where someone who worked in the industry said that for a lot of videos its basically some rich dude with a fetish that just pays to have a bunch of movies or scenes created, because if you have tens of millions then its nothing to drop a like 100 grand to create loads of scenes and pay the actresses you are into or whatever to make them. plus you are investing in the production so its likely you make a bunch of it back. and they said that there were like two dudes who were super into incest shit and just churned it out in the early 00s and then it just was everywhere. and then it became something in demand for whatever societal reasons. and now its here to stay for a while

so now whenever there is a "this is the new hot fetish" and its getting churned out by one studio or whatever its probably some situation like that and then the copycats want that foot/piss/cosplay/whatever money so they chase it, too