The answer, as far as I can tell, is that it makes it easier for Python (and Ruby and Perl) developers to share code and stay on the "latest and greatest" versions of their libraries. I mean, as a library author, what would you rather do? Just create a single egg, gem, CPAN package, or whatever? Or would you rather create three or four versions of the same package (one for Debian-based systems, one for Fedora-based systems, one for Gentoo, one for Arch, etc.)? I understand that it can be frustrating from a sysadmin perspective (pip, especially, isn't sysadmin friendly at all), but from the perspective of a library author, eggs and gems are much preferable to debs and rpms because I know that they'll work in a reasonably distribution independent fashion.

EDIT: I know that's not a satisfying answer. But, as far as I can tell, it is the primary reason that runtimes like Python and Ruby include their own package management rather than falling back on the package management that the OS provides.