The US Social Security Number (SSN) was first issued during the 1930's as part of President Roosevelt's "New Deal" program, a response to the Great Depression. The purpose of the 9-digit number was to track a persons contributions and benefits under Social Security.
It was not intended to be "personal number" or identifier in the way we now use it - and it shows. It's apparently way too easy for it to be stolen, sold and otherwise mis-used.
Following the clusterf*ck that was the Equifax breach, the Trump administration is looking for a way to get away from using the SSN in this way, and come up with another method of identifying consumers, probably based upon cryptographic data - something like blockchain, already used by other countries.