Exactly. There’s already an entire instance dedicated to nsfw (Lemmynsfw.com). I don’t see why any other instances need to host that stuff. It just exposes them to liability. Better to have it centralized into one instance anyway.
That’s not to say that instances shouldn’t allow nsfw if they want to, I just don’t see the point.
While I don't think every instance needs to host everything, I think the most popular instances being a bunch that don't host NSFW and one that only hosts NSFW is an issue. I don't blame instance admins, as NSFW content introduces additional difficulties, but it still doesn't necessarily look the best.
I know the going advice is "choose a medium popularity instance" which I did, but it took me a while to find one that allowed NSFW but wasn't NSFW-only.