I would say it's probably because there was one or two pieces of media that essentially established netori as a genre, and they used that specific setup. Much like how JRR Tolkien wrote the Lord of the Rings and now pretty much every fantasy story has orcs and dwarves and an ancient evil once thought destroyed but is now coming back and must be stopped. Or for a porn game take, it's similar to when Monster Girl Quest came out and then suddenly every game with monster girls in it was all about how they need semen to live and how good it tastes to their... vaginal taste buds or whatever.
It's not that either of them invented those ideas or were the first to use them, but rather that they happened to use them and then ended up being super popular. For example, Monster Girl Encyclopedia easily predates the release of Quest by a couple of years, but MGQ was the one to blow up (at least in western markets) and neither of them invented the idea of sexy female monsters anyway. In your case, I'm not some netori historian so I couldn't even begin to say which manga or nsfw Super Mario fanfic is to blame, but it's out there somewhere.
There's also a few reasons to use that setup from a writing perspective, but most of it boils down to making it easier to write motivations or using the same tropes as an incest game.
