It's not python that's the dependency that needs managing. It's pip and the keystone cops it brings along.

I'd presume it's quite common with NixOS. At least I don't have python linked to my environment. It might be different would I use the REPL, but I do not, so for me python is a program (or script) dependency, not something I need to carry around. It's actually quite common for many setup scripts to fail when python is not installed, but not all of them list python as a dependency either.