Not to make light of your concern, but what would you have them do with all the dollar bills that pile up in this "lock box"? If you or I were a fiduciary for a trust, we would owe an obligation to the beneficiary of the trust to invest the money prudently. Social security works the same way; the money is invested in special United States government obligations, which have always been viewed as the touchstone of prudent investing. In fact, for social security to remain solvent, it relies on the interest paid on these special notes.
Here is a link that makes the mechanism clearer.
Social Security Trust Fund - Wikipedia, the free encyclopedia
Now, you and I may legitimately gripe that the obligation of the federal government to repay those social security "special notes", when added to all the other government obligations, is too large. But the solution to that problem lies in fiscal discipline throughout the government. The mechanism for funding social security is not the actual problem and changing it to a warehouse-sized "lock box" full of dollar bills is not the solution.