This is kind of a wild guess and it might mean little outside of a specific context, but I feel like it holds true at least for hentai involving popular characters: people these days probably see "vanilla" as just another flavour of NTR in some way or another, either consciously or subconsciously, because in the end you're still sitting in the cuck chair in front of your screen watching another man have his way with a girl you like. So, might as well go all the way. Some will mental-gymnastic their way into "self-inserting", but deep down they know it's kind of a cope. Might be the case for a good chunk of porn overall when you boil it down.
Kinda semi-related, but this is also part of the reason why I've personally come to consider the specific subset of dance-only MMDs to be vastly superior to anything else in the space in recent years: no male means all of the above goes right out the window. The girls are just dancing for the real you behind the screen, and it's as shrimple as that. But again that's just my opinion.
Thank you for coming to my TED talk, lol
