Cause ntr means dragging around characters that no one cares for.
thats because people who make ntr games, and porn games generally, don't know or care about the fetish and are just making it because they know people like ntr , or worse know they call their game ntr among other things because their game lists every fetish ever.
ntr is supposed to have some sort of personal reason why she shouldn't have sex with other men tied to the husband , son , friend who has a crush on you etc, that isnt strong enough to contest with her own lust, and/or need to mate upward.
its like if you had incest porn but the characters didnt look like each other and you only know theyre related because someone at some point said they were. it would technically be incest porn but it wouldn't meet the criteria needed for it to really be incest porn.
edit: i spelled to wrong.