It's an artifact of the unregulated credit system. Credit reporting agencies starting using SSN as an identifier for consumers, thus it gets pasted everywhere. Back before the financial crisis, there was little to no oversight. If someone got your SSN and some basic info, they could open up credit accounts in your name very easily. Most upstanding places like Banks would require some form of ID. But what about the cashier at K-Mart when you were filling out an in-store credit application? They were much less caring. Same with used car dealers, and tons of other places.