To be fair...
There are laws that force Social Security funds into government bonds. The concept of Social Security was economically perverted to start with, but that was the solution for the trust fund that would do the least damage to the rest of the economy.
So where else would you put a government "trust fund?"
God help us all if the government had been allowed to invest trillions under its direct political control in the private equity market. They would eat the entire private sector. Any future privatization effort that starts going in that direction (government buys stocks with the money) should be nipped in the bud.
Buy gold with it? Yeah, right.