pip: pip install ./path/to/some.egg doesn't work

Partially I’m just curious, but I also think this should probably work: why does ‘pip install [sdist]’ work but not ‘pip install [egg]’?

The ability to locally install binary eggs would be helpful when distributing software to end users for testing, before sticking it on PyPI.

(Sorry if this is a duplicate, none of the tickets I could find involving eggs seemed to cover this exactly.)