Except it doesn't. At least not for a large swatch of users.

10 years ago, what I wanted in Python was pretty much entirely in the stdlib. The contents of stdlib haven't changed that much since then, but MY needs have.

For what I do personally, a distribution without NumPy, Pandas, Numba, scikit-learn, and matplotlib is unusably incomplete. On the other hand, I rarely care about Django, Twisted, Whoosh, or Sphinx. But some users need those things, and even lots of supporting packages in their ecosystems.

What makes a "complete distribution?" It really depends on context. The stdlib is an extremely good compromise, but it absolutely is a compromise. I feel like there is plenty of room for different purpose-driven supersets of the stdlib to make different compromises. Steve Dower lists 10 or so such distros; what they have in common is that SOMEONE, decided to curate a collection... which does not need any approval from the PSF or the core developers.