Because you're using an payments processor to collect payment, we need to ask for your personal information, like your Social Security number, DOB, and address, to verify your identity. Essentially, the payments processor needs to ensure you are who you say you are and that your financial activity isn't suspicious.
This is to comply with anti-money laundering and Know Your Customer regulations, and to mitigate fraud risks, which is mandated by laws like the U.S. Patriot Act.