Because NTR is basically a fetish of the most hardcore of masochists. Rather than just having a dominant/sadistic woman, they want to feel shameful and be absolutely humiliated as they are treated like trash and thrust into the depths of despair with no escape and no recourse. The sex often isn't the important part, it's meerly a means to an ends, the key part is showing how much better the other guy is at sex than you; which is why it is depicted as so good that she completely loses her ability to make rational decisions.
Here's the problem with defining NTR this narrowly: On this site it's not only people who like the kink or make/upload the kink that get to tag it... More often with NTR it seems like it's the haters pushing for things to be tagged, so their precious egos aren't exposed to it.
I don't consider myself a fan of NTR the way you've defined it, narrowly and shame-based, but a lot of the stuff that I
am into, like consenting non-monogamy, corruption/sexual-awakening, polyamory, etc is enough to trigger a subset of the population here, so it gets lumped in with NTR all the time.
So if the goal of the haters is to define NTR as broadly as possible, so they can push anything remotely like it out of their bubble, you've gotta think that the people who
do search for the NTR tag, or at least don't actively exclude it, have a range of interests that might not be quite as narrow as you'd like if you're one of the "hardcore" NTR fans.
For example: I can easily imagine a scene in a game where an MC's wife fucks the poolboy while he watches on the security camera, then later that night she'll tell the MC in lurid detail how she got ploughed by that virile young stud
while they're also fucking. Not to shame him, but to get him even more excited.
I mean, that deliberately doesn't match your own definition of NTR, but it would 100% cause some folks here to demand that it get tagged as NTR... You see what I mean?
So the question for a dev making NTR content is gonna be whether they stick to their own specific interests, whether that's true NTR or just something else that got pushed out of the bubble, or whether they explore other related kinks because people have asked them to?
Personally I don't think there's any harm in asking a dev to explore something different or tangential, as long as you're prepared to take "No" for an answer... But by the same token I've seen NTR fans get pretty tribal when a dev does that, accusing them of caving to pressure or similar nonsense, and you're only harming yourselves when you do that... If a little broader appeal gets more people into a game, that's a good thing. If a detour from expectation helps a dev to identify new interests of their own, or just gain a new perspective on their work, that's a good thing. Hell, even if a dev 'sells out' for the Patreon money, so they can maybe quit their shitty day job and still pay the rent, even that's ultimately a good thing.
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Now, to answer the OP's question, given my place outside the bubble, but not strictly within NTR fandom:
I'm a giant fucking carebear, so I like games to end with all the characters being sexually fulfilled in their own way... That is, not the player's way, but the
character's way. Bring me on that journey... So say, if the character's really into the shame thing show me that by finding it's ultimate zenith. You can have the shame spiral into sissification, or have the character transform themselves into the very image of Big-dick Alpha Male perfection trying to escape from it, only to realise it was that shame that got them off all along, and they dive right back in. Anything that's not an unrelenting downward spiral ending in oblivion, or a pointless journey that just ends right back where it started.
But one of my holy grails would be a game where I have to work to encourage the MC's wife/girlfriend/lover to fuck certain other people, not just the MC... You'd be doing all the 'relationship quest' stuff, only not for the usual self-gratification but to watch your partner blossom into a true sexual dynamo. Stuff like building up the nerdy neighbour kid's confidence enough that he'll actually ask her for sex as his 18th birthday present. Or you really want her to fuck her boss but she thinks that could impact her career, so you have to create a setup where the boss fucks her without knowing who she is. That sort of thing.
The MC can have sex with her basically whenever he likes. She wouldn't be cucking him. But all these new experiences would unlock new scenes for the two of them to share together. And of course it would have some group sex and MC voyeurism sprinkled in there.
I wouldn't personally think of that as an NTR game. Hardcore NTR fans definitely wouldn't. But of course the vocally anti-NTR folks absolutely would.