> Suffice to say, is there a significant reason to not allow it?

It's poorly supported by packaging. In particular, relocating an environment isn't supported with entry-point scripts, which pip installs with a fully-qualified shebang. Moreover, entry-point scripts in Windows are created as exe files (e.g. "pip.exe") that embed the fully-qualified path of python[w].exe in the environment, plus a zipped __main__.py. For example, given an environment at "C:\Temp\env", running "C:\Temp\env\Scripts\pip.exe" in turn spawns a child process with the command line: "C:\Temp\env\Scripts\python.exe" "C:\Temp\env\Scripts\pip.exe". This breaks if the environment is renamed or relocated.