Why is Stripe asking for a SSN (social security number)?

Stripe is bound by federal laws and regulations to collect a certain level of information about folks who are signing up for the platform. As they work in financial services, this is very similar to if you were setting up a new bank account at a new bank.

Your credit score is not affected by entering only the last 4 digits of your social security number.

Please see this article for more details via Stripe: https://support.stripe.com/questions/date-of-birth-and-last-4-digits-of-social-security-number-ssn-requirement-for-us-stripe-accounts