If building a retirement fund is left to the individual as the USA is becoming a corrupt society (giving evidence to the fact that even a great nation can be taken over by a few ruthless & turn it corrupt) where few people control most of the wealth (including the media) then all the money in retirement funds of common people will end up in the hands of the few at the top since the financial market is becoming more and more a zero-sum game where an average person has no chance of going mano-a-mano with the powerful insiders of the financial markets .
In addition, most people cannot afford to save enough to invest for a private retirement fund because they are not paid an adequate compensation to save when they have a job in some cases and in other cases because there is no security of an ongoing jobs, with large corporations (own by the 2% at the top) exporting those jobs to make more wealth. The greatest American of the last few decades Franklin D Roosevelt (who dedicated his life to serving the common people, not the 2%) knew this and this is why he created Social Security Insurance where both employers and employees by law were required to set aside a certain amount as insurance for the retirement fund of employees.
When FDR created Social Security trust fund he anticipated that those funds to be separated and invested in government bonds paying a fair market rate for money. FDR did not anticipate the future politicians to be so uncaring as to tap into the retirement funds of the least powerful members of the society and destroy the only nest egg they had. FDR apparently did not have the foresight to see the rise of Nixon, Reagan, Bush, Burns, Greenspan, Bernanke and Fox news (Murdoch) to take over the USA--still Bernanke and Murdoch have time for redemption.
The Social Security Trust fund is going bankrupt because politicians stole the trust fund money to give as corporate welfare to their cronies and second because the Federal Reserve brought the rate of interest so low that the trust cannot make a fair return even on the money left in the trust fund. The Fed brought this rate so low also to help their cronies who control most of the wealth in the nation. It is only those few at the top can borrow money at those low interest rates but as you go down the ladder the poorest pay legally-protected usury rates to borrow money. Also let’s not forget even the trillions in new money Fed is creating also end up in the pockets of this 2% without tricking down except as debts to keep the 98% in further bondage and fear.
If you look at the budget you will see that most of the expenses in the budget are due to corporate welfare programs (two biggest expenditures defense and Medicare are replete with corporate welfare). Social security check is the only one that goes to the benefit of the common people. This is why the rightwing is hell-bent on destroying social security since they detest any benefit to the common man but want to empty the treasury as welfare for the 2% at the top and have the 98% available to bargain for slave labor to make more money for the 2%. Capitalism is a great economic system but what we have had in the USA since the 1970s is not capitalism. This scam has been possible only because the wealthy few have managed to brainwash a significant segment of the population through extremely well-paid media personalities and keep the good people (for example good, intelligent people such as Noam Chomsky) away from the media. If people knew the truth then this scam would not have been possible.