A Fonte

"Everybody goes around talking about the problems created by the declining number of workers per retiree," he said. "How come life insurance companies aren't in any problem?"
[…]
Life insurance companies take premium payments and invest them in factories and buildings and other income-producing assets, Friedman said. These accumulate in a growing fund that can then pay benefits. Social Security, by contrast, operates pay-as-you-go, collecting payroll taxes from workers that immediately go to pay retirees.

Milton Fridman