You aren’t promised your money back with interest; that’s what an IRA is. Social Security is a different system which works by different principles. In Social Security, the money you pay goes to the people who are retired, and your retirement is funded by people who work in the future.

Your money --> older generation

Younger generation ---> you

The main thing that makes my description incorrect / incomplete is that baby boomers, such a large cohort, broke the system. This was predicted and so the baby boomers actually did get their money back: they were taxed to fund OASI, which then goes back to fund baby boomers’ retirements, until it runs out.

Baby boomers (only, sort of) <---> massive cushion in the OASI fund