Pip + setuptools is "it's working, no need to change" in the same way that "SVN works so why bother with Git?". It seems fine until you try the alternative, and then the thing it replaces just feels painful to use.
It certainly has issues of its own, and I'm sympathetic to people who prefer the UI of alternatives like Mercurial. But wow, I'd never go back to svn (or god forbid, cvs) ever again.