Why are Central Asia and Southeast Asia ideal places for growing rice?

They are ideal because agriculture has been the main economic activity in China for thousands of years. About 60% of all workers in China are farmers. Nearly all of China's cropland is in the eastern half and the country. In southern China, rice, sweet potatoes, and tea are major crops. Wheat is the chief crop in the north.

Besides that point seasonal monsoons bring the amounts of water needed for growing rice in those regions. (Rice needs to be submerged in water to be grown)