It's basically because SSNs were not supposed to be used for that purpose, they were only supposed to be for the SSA to track people, like an account number. I

It's basically because SSNs were not supposed to be used for that purpose, they were only supposed to be for the SSA to track people, like an account number. In fact, the paper cards used to say "NOT FOR IDENTIFICATION" on them. I forget when that was removed. The military started using them for ID in the 60s, then the IRS started using them in the 70s, and it's just kind of morphed into an ID number because "everyone has one".