Why does pip freeze list "pkg-resources==0.0.0"?

According to https://github.com/pypa/pip/issues/4022, this is a bug resulting from Ubuntu providing incorrect metadata to pip. So, no there does not seem to be a good reason for this behaviour. I filed a follow-up bug with Ubuntu. https://bugs.launchpad.net/ubuntu/+source/python-pip/+bug/1635463

I had the same problem in my virtual environment. I removed it with pip uninstall pkg-resources==0.0.0