The main reason is that the developed world is developed. There are existing cities that attract new people. Cities need to have a reason, midsize ones have pos

The main reason is that the developed world is developed. There are existing cities that attract new people. Cities need to have a reason, midsize ones have position and big ones are big. A new greenfield city has neither of those. The US has been making new cities, or at least growing smaller ones. Many of the metros in US like Phoenix, Miami, Tampa, and Orlando were small 50 years ago and barely existed before WW2. There are lots of new cities in China. Shenzhen was a village 50 years ago and has 17 million people today.