A fetish is a fetish, there are no big reasons to think about it, but indeed they do tend to have different reasons as for why they enjoy it. One reason I heard, as DragunsBahamuth said, is that feeling envy spices it up for them, but I also read from some people that it's the degradation that does it for them.
Some people play NTR games because they have better art, or because they have other fetishes that more normal games don't have. As an example: Himawari wa Yoru ni Saku, it's one of my favorites, is more of a Netori hentai but it's rare to see a Vanilla one with the same Quality. There are still Netorare vibes but I can more easily self-insert with the UB than the Cuck, and at that point it's the good version of NTR where the MC does the cucking.