Why aren’t we adopting a one-child policy?

I have been thinking about this for ages. Would love to know other people’s opinion on this. We are 1.4 billion people. Except china no other country has reached this mark and they at-least tried to stop it before 2000s with a one child policy. Looking at the amount of people in metro cities, pollution I definitely think there should be a one child policy after a certain year. I know for sure that people in metro cities are either sticking to having 1 child or no children, but its not the case in rural areas. They still do feel that more children will solve their money issues altogether.