They have a cultural expectation that their children takes care of their parents. That also applies to the parents as well. In the worst case, one kid could be taking care of 2 parents, and each of their parents as well. The kids are still the retirement plan.

Glass shatters

Stone cold music plays

40 decades of one child policy takes the stage

And yeah I know the one child policy has been over for years but it set a cultural expectation and they built their infrastructure around it. Schools, daycares, leave, benefits, plus the strongest factor of all: people try and have families similar in size to the ones they grew up in generally speaking

Edit: and as to how that hurts a nation, it's really hard to save money and grow your wealth as an individual when you're working your butt off to support 6 other adults by wiring them money into the countryside. It prevents you from starting your own business (too much risk, not enough money or time) and is a giant economic drag on the economy. We're already worried in Canada about retiree to worker ratios and we're nowhere close to 6:1. I think it's something like 1:3 for us right now?