It's a pyramid scheme. You put in $10 over the course of your career, but you take out $20 during retirement (funded by the next generation(s)). If US had shrinking population like Japan instead of growing population it would quickly collapse on itself because each generation takes more than it gives and relies on population growth to not collapse on itself. Or maybe social security rules would change so you have to be 75 before you get any benefits so that most people die before they qualify.