to offer insurance, you have to be able to pay out if a claim is made. Since the insurance you are offering is all or none ( i.e. if social security goes away, it goes away for everyone ), you would have to pay out claims to everyone at once. This is much different than auto insurance where you only expect some small percentage of policy holders to put in a claim.

You need to find someone to take the other end of the bet. So, who would want to insure themselves in the case where social security stays around? Can you find enough of these people to balance out your risk?