The Ponzi scheme

While the vast majority of retirement plans are run responsibly, unethically using workers' retirement contributions to fund current expenses, instead of saving them for workers' retirement, is still an all-too common occurrence.

By far, the biggest perpetrator of such shenanigans is the US government.
Unlike responsible plan administrators, which save all retirement contributions rather than spend them, the government immediately spends most of our Social Security contributions on current retirees. Were other retirement plans to do that, it would constitute an illegal Ponzi scheme, in which money from new investors is used to pay off old investors.

The Social Security contributions that do not get spent on current retirees are commingled with general funds, going toward government salaries, departmental budgets, pork-barrel projects, foreign aid, and the many other things that the government spends money on. Not a penny of it is ever saved for workers' retirement. In the private sector, that would be grounds for certain conviction.