That sounds all rosy and nice but unfortunately that's not how it works.....
The government borrows your Social Security money, they issue a bond in place of said money. Then instead of KEEPING THE MONEY, they spend it thus leaving a GIANT HOLE where that bond used to be...because not only now do they have to pay interest on that bond THEY ARE IN THE HOLE for the face value of that bond.
Now...if you go to treasury direct.gov and look at the definition of "intragovernmental holdings" you will find that this is where the money borrowed from Social Security is placed AS PART OF THE NATIONAL DEBT to the tune of nearly 5 trillion dollars.
That is how all government debt works. All government debt is money that is raised by the government then spent by the government. Or almost all of it, anyways.
All pension funds have some portion of their fund in government debt. They lend their pension monies to the government who spends it. A government pension fund does the exact same thing. It lends to the government who then spends it. The only difference between a regular pension fund and SS in terms of its government holdings is the instrument of debt owed by the government. For a regular pension fund, it holds government bonds. For SS, it holds nontransferable obligations of the federal government.
The biggest problem with SS is that the trust is made up entirely of these types of obligations. I cannot think of another pension plan that is like this. It probably violates ERISA, and no pension fund consultant or professional would ever design such a fund.