It's not just that, it's also a filesystem layout issue. If you install everything in `/usr` or `<venv>/lib/pythonX.Y/site-packages` you cannot have two versions / variants of the same package installed concurrently.

For that you need one prefix per installation, which is what Nix, Guix, and Spack do.

The runtime can also use mount namespaces to support concurrent installations. Or, if there is a compilation step, the linker can not expose symbols for clashing libraries and just resolve them within the dependency chain.

The package calculus allows all of these to specified cleanly in a single form.