My question...
As someone entirely outside the NTR fandom, my guess is there's no non-Freudian answer to this. There's some weird social/psychological baggage around men and their mothers; why else is mother****er such an ingrained insult, or "I did yer mum" such a common attack? The act of sleeping with a man's mother is often interpreted as an act of dominance over that man, and I suspect that's Mommy NTR people are responding to. That Ugly Bastard isn't just piledriving your mom, he's making you less of a man by doing it.
And of course for the VN crowd any mother in a game is assumed to be a love interest, and many players assume that any
potential love interest might as well be committed to the MC at game start
I mean, have you read the comments under some incest games? I've seen people pitch an absolute fit because the main character's mother is stated to be sexually active with his father (heaven help the dev that actually
shows a scene between the MC's parents). So yes, some people unironically consider it NTR for their dad to have sex with their mom (at least in the context of a VN... I hope).
To answer your very complicated psychological question "Why NTR?" you'd probably need a certified psychologist but I can try to answer it. Keep in mind I am answering this question for myself not for the whole NTR community, everyone has their own reasons.
I really like Femdom and NTR is in a way (for me) an extension of that female domination fetish.
(idk if I could answer the "Why femdom?" question if you'd ask. Guess I just really love anime girls that much)
It's more like NTR/Netorase is a smaller branch within the huge concept of Femdom. Personally I prefer Netorase but I like NTR as well. With Netorase the wife is in full control of the sexlife and chooses if the protagonist has fun or not while she can have fun with or without the protagonist on a whim (and she does). In Netorase there can be an incredible amount of teasing as well. Where the husband knows about it but doesn't always know the when, the where, the how and with who. If you play as the teacher in the game she could be fucking one of your students and it would be like humiliation in a femdom game except she's getting a lot out of it as well, she has the time of her life. It gives her an incredible amount of control not only within their sexlife but also controlling the well-being of the husband by choosing how much she'll shock him or spare him with a new picture, video or just leave him haning with a very suggestive joke or text message.
In Netorare all of this applies as well except you play as a clueless husband who the wife is humiliating even more because she is also outsmarting him by not getting caught, she dominates him in intelligence as well since outwits him by getting what she wants when all the while what she wants is a forbiddenfruit... (for me at least since I prefer male protagonist in NTR and Netorase over female protagonist)
While having sex she has the time of her life while the husband is either kept in the dark or has to settle with the scraps she throws him and be happy like allowing him to cum on her soles while she's giving a blowjob to the other guy or having the husband clean her out by licking her cum filled pussy after she comes back home from a party while the husband was at home vacuum cleaning and doing the laundry (if it's Netorase).
The rush of getting caught some people enjoy.
The mystery of not knowing, some games let you play the husband PoV first and you barely see any sexscenes. After you finish this you can play the wife PoV and find out how the most wild things were happening right under the husband's nose when there was sometimes even a mere thin showercurtain or a blanket in between him and the most degenerate love making ever. There is a jealousy towards the wife but you still love her so incredibly much, it's just so unfair how she teases you like this...and she can. However you shouldn't even think about cheating for a second, there would be trouble and consequences.
(While writing that part it made me remember how the giantess fetish is also surrounded around femdom and how people love the kink of the size and power difference between a 0,5mm (0.019 inch) male and a normal sized girl which from the shrunk male's perspective is so tall and powerfull it might as well not be a human but a goddess, it's the unfairness some people like and this unfairness between female and male is prominent within femdom)
In both NTR and Netorase the wife can be very loving and caring she really loves her husband but sexually she just needs more sometimes and in NTR she takes what she needs while in Netorase she knows the husband gets off to it but gets carried away sometimes making her do questionable things. The husband is okay with this because he wanted them both to have fun not just him enjoying the fetish.
There is also look at the third party which is the guy(s) the wife is cheating with for me the perfect candidate is someone who the protagonist should be outmatching on every point, physically, intellectually, financially and status wise, age wise. Yet he gets to have absolutely animalistic sexual mating with your beloved lovey-dovey wife. This for me makes the Netorase and the Netorare incredibly humiliating and again playing in on the femdom since the female humiliating the male is a common thing in femdom.
But when the guy who she's having sex with is better than the protagonist in all these above aspects, then he's like a wall between the wife and the protagonist. Even though she is your wife you should have unlimited access to her but now she's like a beautiful jewel on display in a highly secured museum while you're like a visitor gawking at her magnificence... but that's not fair she's your wife! She has access to something like this while your access would be met with a very strong response. There is a power and freedom disparity between the wife and the husband but you love her so much you want her to have this favorability since that is how much you love her even if it's destructive to oneself. To some this is even romantic.
On the opposite side of this are people who don't like this at all. It's understandable for example if someone had a rough breakup, bad releationships or a cheating wife IN REAL LIFE, of course this fetish would be horrendous, is also why I'd try not to ask people "Why not NTR?" because it's an easy and understandable question to answer but people do often ask "Why NTR?" and this seems more complicated and my attempt to answer this wouldn't come close enough to explain.