Aj, prebuilt packages from PyPi and the impure “venvs” are a separate issue. PyPi distributes software built for FHS platforms, it does require host configuration (nix-ld, nixglhost, etc in case of NixOS, or “apt-get install”-ing “system dependencies” in mainstream distributions).
When creating a venv also make sure to pass the flag telling it to “use system dependencies” (may not work in older nixpkgs versions, cf Fix venv creation in Python environments by cwp · Pull Request #297628 · NixOS/nixpkgs · GitHub)