What if the user doesn't have pip installed? This is before we've installed the requirements, so we shouldn't be assuming anything outside the stdlib.
from my understanding, when you build the package, your computer extracts the relevant required packages, and sends up that data to pypi, which then in turn sends that data back to users when they install
At least thats how I think it works
if you used any other package manager, it would need to resolve dependencies from the additional info in the package index, resulting in pip being downloaded and installed before the desired package is installed