What this accounting sleight-of-hand does politically is to confuse the public and maintain the illusion that your Social Security "contributions" are being put aside for your pension when you retire. That would be true if you were investing in the private sector, but not in the government.
The law requires private insurance or annuities to be backed up by enough assets to cover their liabilities. Private insurance companies can't just promise people anything and leave it to someone else in the future to figure out how to pay them off -- or how to evade the responsibility for paying them off. Only the government can do that.
There is a reason why you don't hear the same hysteria from private financial institutions that you hear from the government about how hard it will be to pay the baby boomers' pensions when they retire. Private pensions are financed by investing the money that comes in, so that the assets will be there when retirement time comes.
Forget about money for a moment. The real wealth of the country consists of goods and services. When you add to those goods and services, the country has more real things available for its people's use, including use by people on pensions. That is what happens when private pension funds are invested in tangible assets like buildings and factories -- and what does not happen when the government runs Social Security and spends the money as fast as it reaches Washington.
Private pensions belong to you. Government pensions are controlled by politicians who can change the rules any time they want to. That is what makes Social Security a risky