I don't think you understand how Social Security works. 

We spend our whole lives paying into Social Security. When we retire, or if we are disabled in life, then we withdraw. It's not a handout the young are paying to the old. It's getting back the money that you have already put in. 

YES, it's being used, not just sitting in a vault---but the amount coming in isn't the same as what is going out. Same thing with your taxes and refunds. That money is used by the government---often in investments. It's a free loan we are all giving to our government, expecting to be paid back. 

Given that, it's up to the government to figure out how to pay us back, rather than just blowing it all. Of course, governments seem to be very bad at this.