I’m a little surprised at this point that Apple still doesn’t include a default Python 3.x on macOS. It’s the single thing keeping me from moving (as there’s a big difference between “just run this” and “first download this, then run this”).
I agree, though it's not just macOS: all of the BSDs are behind on Python 3. Once FreeBSD gets Python 3 by default, I bet macOS will follow.
Current FreeBSD Python 3 status: https://wiki.freebsd.org/Python