Why is Stripe is asking for my social security number?

A date of birth and the last 4 digits of a Social Security Number are required as part of Stripe’s mandatory identity verification requirement: Stripe must collect, verify, and maintain information on the individuals associated with every US Stripe account, as required by their regulators and financial partners. This is intended to promote transparency and prevent individuals from using complex company structures to hide terrorist financing, money laundering, tax evasion, and other financial crimes.

Stripe's Privacy Policy explains how and for what purposes they collect, use, retain, disclose, and safeguard any personal data you provide.