Comment by zimpenfish

> companies started using SSNs like an identity verifier

Probably because USGOV said it is[0]

"In 1943 a presidential executive order directed the military and other government agencies to use the number for identification purposes, and in 1961 the Internal Revenue Service began using the number for taxpayer identification."

[0] https://www.straightdope.com/21341325/why-does-my-old-social...