I mean those pensions are typically invested in vehicles that provide growth to keep up with inflation etc, and that growth is largely powered by the workforce who by the time you retire are your kids. Or in your case other people's kids I guess?
https://www.mckinsey.com/mgi/our-research/dependency-and-dep...
https://www.cato.org/cato-journal/spring/summer-2018/demogra...
https://www.cato.org/cato-journal/spring/summer-2018/demogra...