This totally reminds me of SSN use, when initially they were just a number (not secret) to identify a person, and then suddenly people started to use them as a key for authorisation, because someone had a bright idea how to implement things fast/simple/cheap (cheap part comes at expense of others)
Rather than being about fast/simple/cheap, I think using SSN as a key was more about the fact that SSN is the only common identifier that almost all US citizens have.
I think you're using the word "key" differently than OP. You're talking about identifiers, and they're talking about security.
SSNs were a good potential identifier, until the people that needed security cheaped out and started using SSNs as a bad implementation of security. Now they're bad at both purposes!
Yes, designing and implementing a new common identifier almost all US citizens have would have been less cheap and fast.
Tax ids were never meant to be used as a form of global identification. If you go look in a real bank core, you'll find this field does not have any uniqueness constraints.
Why not? Two people with the same tax ID seems like a problem waiting to happen.
These schemas also support non-individuals with the same fields. EINs have a lot more edge cases than SSNs.