Thank you for such a long and detailed post! I indeed did not know about things beyond the SIM, and I didn't know about the extra details about the country codes either. That is extremely interesting to me.
With the phone spoofing though, does that mean two factor with a phone number is basically useless? If I had authentication based on a MAC address, it would take seconds to break it. But I think, and sure hope, that auth based on phone numbers is more secure.
I think your domain name answer -- that for the most part computers didnt need them -- is a very satisfying answer.