Why? You can figure out someone’s SSN if you know where they’re from and what their birthday is. They’re highly predictable - particularly given that you’ll often see the last four digits as the “obfuscated” version in lists etc, and the first three are state, and the next two are based on when they were born. Put the two together, and job done. They’re effectively public data. The fact that they’re used as a magic security number/identifier is frankly mind-boggling.

Citation: https://www.pnas.org/content/106/27/10975