With a shrunken generation of family to help due to 30+ years of one-child policy, and almost no social support (China does not really have socialized healthcare in practice) there's no one to help take care of larger families. But there is a HUGE number of rapidly aging parents and grandparents to take care of.
Add a culture that puts accumulating wealth as the #1 virtue, and it becomes nearly impossible to reverse a low birth rate.
In Canada Quebec has been paying French families to have more kids for a very long time. Today Quebec has the lowest provincial birthrate in Canada.