It's really not that complicated. You don't have to go into some "psychological, sociological, and paleo-anthropological" about it. It comes down to whether you self insert or not. I've always had this assumption, but made some polls recently to confirm it.
https://f95zone.to/threads/poll-do-...ay-games-with-a-third-person-mentality.89596/
https://f95zone.to/threads/poll-ntr-fans-self-insert-roleplay-or-third-person.86527/
The first poll is a bit premature, but there seems to be a trend already. I know the sample size is pretty small at this time, but lets just say 40% of the overall porn game player base prefers to self insert. Now, I don't have numbers, but I will make an assumption that true masochists make up a very small percentage of the population. Thus, the people that self insert will overwhelmingly dislike NTR, since most people don't want bad shit to happen to
them.
The poll aimed at NTR fans have a wildly different skew. Only 10% of them picked self insert. These people would be the true masochists. Note that even in a curated sample of just NTR fans, there are very few masochists that want to experience all the negative emotions in first person. 80% of them are third person viewers. I am usually one of those and the appeal is mainly in the corruption, depravity, and taboo nature, much like many other fetishes.
So coming back to the question of why NTR is so popular. I don't think this is a true statement. NTR fans would be a subset of the ~40% of the overall population that prefer to view games in third person. I'd make a guess that even third person viewers mostly don't like a really negative and fucked up story that is NTR.
BUT. If we accept the statement that NTR is popular, it would be because it is taboo and depraved. Girls cheating and betraying their loved ones. This holds true especially for non-game media, since it's rather hard to self insert, or even identify as the cuck. If we ask "why is there a rise in cuckold porn videos?" The answer wouldn't be that there is a rise in the cuck population. It would be because people just find the fucked up stuff that goes on in these stories hot as a third person viewer.