I think because of two main reasons, the first is that there simply aren't enough netori games to justify a tag. The second is that most people on this site don't understand what does and doesn't constitute NTR.
Honestly, I have seen this argument for quite a long time now and it frankly just feel like they are being stubborn at this point.
So let break down your points:
1. "There simply aren't enough netori games to justify a tag."
There are more than enough on this site if you take the time to go through them in my opinion, and it will only increase with time. There are over a thousand games on this site with the netorare tag, so even if 1/10 of that have netori element, it would still be around a hundred games. And there are tags with fewer number than that.
2. "Most people on this site don't understand what does and doesn't constitute NTR."
Well those people sure as shit isn't going to learn about it if nobody bothered to separate the tags for them, now is it? And by splitting the tag, it will FORCE them to figure out which is which. And how? Well, just ask. I have seen a handful of people asked what is netorase or netori in some NTR game threads, and people pretty much immediately tell them.
And honestly, I don't really need the tag for it, just the proper clarification in the Genre or Uploader note. Some of them does elaborate (bless those fellows) but most just say NTR and leave it at that. Which of course start the "Is it avoidable?" and "Which NTR?" train again. Every time.
