> The simple test for that is, why do people use SSNs as it's not legally required for anything but USG interactions.
Because if I want to open a bank account, rent an apartment, or get a mortgage, the other party requires me to give them my SSN, on the assumption that it is a valid unique key for tracking debt payment reliability.
It doesn’t matter that the SSN is only legally required for interactions with the US government. When housing is a human necessity, and all sources of housing require a SSN, then using a SSN is not a choice from which one can infer preferences.