> which wants... a social security number because why?

Because of federal child support legislation. If you are $2500 (or more) in arrears, your passport gets cancelled. Most states will also suspend/revoke your professional licenses and possibly driving license when you cross that state's threshold.

https://travel.state.gov/en/passports/contact-support/legal-...

https://en.wikipedia.org/wiki/Child_support_in_the_United_St...

> In 1996, Congress passed and President Bill Clinton signed the Personal Responsibility and Work Opportunity Act (42 U.S.C. § 666), which required that states adopt UIFSA by January 1, 1998 or face loss of federal funding for child support enforcement. Every U.S. state has adopted either the 1996 or a later version of UIFSA.

https://en.wikipedia.org/wiki/Uniform_Interstate_Family_Supp...

When I worked for my state's motor vehicle bureau, one of the verification apis that the driving license/ID folks got to use was a verification of citizenship/lawful residence service. Which used SSNs.