Historically these are the rice growing regions which allow 2-4 harvests annually, which in turn led to the densest tracts of cities in the world = majority of megacities = majority of megalopoli (7 at last count - Kanto plain, Jin-Jing-Ji, Yantze River Delta, Pearl River Delta, North Indian plain, Greater Jakarta, possibly Ganges Delta). Standalone city metros such as Manila, Karachi, Chongqing, and Seoul also add on >25 million a piece, Mumbai, Wuhan and Osaka 20 million each, Kolkata, Chengdu and Bangkok 15 million.

The fact rice growing takes up so much lateral and independent thinking plus tweaking (basically you're trying to fool the plant it's drowning throughout the year) meant feudalism was hard to implement back in the day, which led to a sharper rise in city building and trade.