The Haskell ecosystem seems obsessed with NixOS in a way I haven't seen anywhere else.

I recall wanting to use Haskell packages that were only available in Nix, but nobody had bothered to make just a normal package.

To be fair, that’s a general problem on NixOS (and other OSes), and one that tools like devenv have tried to solve. You can take what’s included with OS packages, but then you’re stuck with whatever version that is.