Enum is just one tool among many, no more special than named tuples, nested dicts, string.Templates, regexes, pickles etc.  

The second issue is keeping the scope of argparse focused on its core task rather than trying to incorporate other parts of the standard library.  That is called separation-of-concerns or orthogonality.  A little parsimony is necessary for loose coupling and high cohesion.  We also don't want module sprawl or feature creep to impair maintainability or affect learnability.

That said, this is up to the module creator and maintainer, Steven Bethard.  He has the most experience with module and has the clearest vision of what its boundaries should be.