Usually, the farfetched plot reason is because MC is one of those types that are super oblivious/heroic/do-googer/innocent/puts-girls-on-pedestal.
But the real (meta) reason for the MC-being-left-out story is generally because doing something else messes with the brutal and unfair style of NTR. If the male MC starts getting corrupted or is cheating himself, it's no longer some huge betrayal when the girls turn into the usual NTR sluts.
Yeah, I'm familiar with it; I've played a ton of NTR games, some of which don't even make sense. The thing is, it's not always 100% accurate; many NTR stories actually use a moment of cheating as the breaking point for the woman to fully embrace being a cumslut.
For example, in one game I played, the wife of Joe, a salary man, was getting involved with his boss. She resisted, sticking to touches and some oral sex, until Joe ended up sleeping with a female coworker. The wife, who was already cheating, used it to justify her actions. There's another similar scenario where the wife was fully into NTR but refused to forget about her husband until he was tricked into a love hotel, and pictures were taken. Again, the wife, who was already cheating, used it as justification to leave. So, cheating doesn't necessarily hurt the NTR storyline that much unless you only see it going in one direction—woman taken from man. Some people even argue that a man being taken from a woman isn't NTR somehow.
But I digress; having a story follow the typical beats of an NTR plot, with the MC discovering what's going on, and instead of killing himself, making it the point where his corruption begins, turning him into a womanizer or even a playboy who doesn't believe in love, would be interesting.
