Because it was used as BOTH an identifier AND proof of identity, for a long time. If it were used properly as simply an identifier, you'd be right, but there ar

Because it was used as BOTH an identifier AND proof of identity, for a long time. If it were used properly as simply an identifier, you'd be right, but there are still many cases where knowledge of the number is used as proof (or partial proof, along with birthdate/address/etc) of identity.