There's plenty of python packaged in nixpkgs too. It doesn't mean that it isn't a dumpster fire disaster. Dealing with it has been trouble with every other distro I've used. It isn't just a nix problem. If anything I think the situation is improved.
Nix hasn't been a benefit when working with python for me, but again, python is the outlier. It has been a benefit for projects in other languages.
I guess the reason is because python packaging/tooling varies wildly between projects, and there are a lot of bindings.
BTW a colleague was setting up the python project on a non-nix machine, and also had problems with dependencies, and ultimately had to do some nasty workarounds (disabling deps/features). To me, it seems endemic.