Because buying a VGC should not warrant the need to give out such valuable information. Its not like you are applying for a prepaid debit card or something and even in those cases the SSN is just used to verify identity. If the manager is not ok with just checking your ID, I would just avoid that store all together. Also since you say you know the store manager, why not just ask her kindly? I am sure she is aware you have bought VGCs multiple times at the store in the past I am sure she would gladly give you the reason.