You aren’t paying attention. Social Security is not a loan. Recipients receive far more than they paid in:
http://www.humanevents.com/2012/02/18/raiding-social-security/
Whose potential earnings are you referring to with your stock comment? Certainly not the government’s—the courts have ruled that there is no lockbox, and you are not entitled to the money you paid in.
SS is an entitlement program, and it’s bankrupt. Not sure why you are having difficulty understanding this.
Regarding the ‘loan’ aspects ~ 100% of everything you pay in FICA is put in a fund where it is then borrowed by the US government. Looking at it from the two tails ~ where it starts and where it ends, it is a forced loan to the government!