> It's the only number we all get, since not everyone gets a driver's license; ID; passport; or other identifier. Unfortunately, it's been used to identify us for everything, and until recently was typically in plaintext on most forms (medical; tax; student; etc...).
I fail to see the problem with that. As you said, it's an identifier, like an username or your full name. There should be no issue with everyone knowing your full name, or your username; why there should be an issue with everyone knowing your SSN, or it being in plaintext everywhere?
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.
I heard there was a similar problem with the bank account number in the US - that you could use it to withdraw money without an actual password or strong identification. Hence the popularity of cheques, PayPal and similar services that weren't needed that much in Europe.
You're right that bank account numbers in the US are insecure, but you're wrong that this is why checks are popular here.
Checks are actually the source of the problem. If you have access to blank check stock and MICR laser toner (both readily available on Amazon, since business accounting departments will routinely print their own checks for payroll / bills), you can make seemingly valid checks to withdraw funds from any account number. This is still a problem.
The reason why checks are popular is because until recently there hasn't been a cheap + accessible + official + unencumbered way to do electronic transfers between personal accounts. The infrastructure existed (ACH), but only businesses could actually initiate deposits/withdrawals. Individuals could initiate full-service wire transfers, but those are risky (there's no way to reverse one done in error) and banks typically charge $25/transfer - which is far too expensive to use for anything routine.
PayPal came into existence so people could purchase goods online (on eBay, specifically) and have the option of performing a chargeback if the goods weren't delivered as advertised.
(Checks will probably still persist for some time, since all the online payment services want to charge percentage fees if they think you're acting as a business. The beauty of checks is that they just work and don't insist on taking a cut of the payment.)
> why there should be an issue with everyone knowing your SSN, or it being in plaintext everywhere
Because far too many businesses, esp. financial ones (banks/credit unions/etc.) have also incorrectly used it as a password to authenticate that "voice on phone" is really John Q. Public and/or that "grifter in chair across desk" is really John Q. Public. I.e., they used the fact that "person X" knew number Y as proof that person X was really person X.
We can argue that it was never intended to be used this way (a true statement), that knowledge of it provides no such proof (also true), and that using it as such was always wrong on the part of these businesses (also true), but the fact is, many did use it this way, and, sadly, many still do use it this way. And it is this misuse that is the "issue" with everyone knowing everyone's SSN.
I fail to see the problem with that. As you said, it's an identifier, like an username or your full name. There should be no issue with everyone knowing your full name, or your username; why there should be an issue with everyone knowing your SSN, or it being in plaintext everywhere?