It will never run out like a savings account could. It is a lifelong benefit that is not reliant on the rise and fall of the stock market.
Beneficiaries receive regular cost of living increases to adjust for rising prices.
It is predictable income. Payments are made to workers and dependents of workers who become too disabled to work. Guaranteed income is paid to surviving spouses and children. Privatization is too risky for the younger worker who needs to build credits now so that they qualify for future benefits. Private investment accounts substitute the risk and insecurity of the stock market in place of the current guarantees.