Your joking right? Eggs, wheels, virtualenv, venv, pyenv, poetry, conda, python2.7, 3.4, 3.5, 3.6 (yes, we have all 4 versions installed in my current company's "production environment", not to mention 3 different versions of python 2.7 but no 3.7)...
I don't have much experience with packrat - but as opposed to pip it's another thing you need to discover and install. And so people don't do it by default when releasing code, and I've had to bisect versions of dependencies to get a working version of code. This can happen in Python too, but is rarer.
Fortunately conda came along and pretty much solved all my problems.