The issue with using pip and package managers in a similar vein is that they're not distributing complete packages, which usually means that stuff is compiled when installing some application. End users don't have compilers, and ideally you'd want to avoid them on servers as well. Often this also needs some or many OS-level depencies, like libraries and headers.

There are of course many other issues with pip and setup.py, far too many for a comment here.

But to give just one example; distutils/setuptools try to play god ^W build system with things like Extension. This works, badly. For example, most compilers that aren't GCC or MSVC compatible are not supported at all. Finding libraries? Not supported. Having proper dependency graph detection of files? Nope. If some "invisible" dependency of an output was modified, won't recompile. Ups. It pretty much behaves like a Makefile written by a naive person, just without the "-jN" flag.