How? How NTR could make things more interesting that any other fetish couldn't? and please don't try appeal to the bullshit about "realism and give MC a challenge".
For the same reason there are people who also hate harem, incest, futa, gay/lesbian, bdsm, femdom/maledom and any other type of content: it's simply their taste, no mystery about that, yeah is just games and not real but people play games to entertain themselfs and have some fun, so it's just logical they don't wanna play something that will not bring fun to them.
Did you also play games like, i don't know, The Last of Us or God of War? Batman Arkham? When we play these game, doesn't we want that the protagonists that we control win instead of of die? It's the same thing about porn game, just that in this case "win" means the MC have sex and get the LI's, not someone else.