> I thought in Nix you can have that, and python can have venv for independent library versions, is it that no one has done the work to combine the two ?

A python environment is effectively a different take on a venv. Python packages is one giant python environment. For programs that life outside of python packages but use the packages from there we are free to apply overrides how we want. So it is possible to have different versions of a package in a different python environment.