Why do most big countries implement federalism?

Mainly cultural issues. There are few federalism in the eastern cultural circle, such as Japan. Western culture believes in the law of the jungle, and our eastern culture believes in harmony, so the starting point of the west is to assume that people are evil, and the starting point of the east is to assume that people are good. In western history, it is a civilization that destroys a civilization, but we are tolerant of it. For example, vassals and tributes can be saved, and it is not necessarily a life-and-death situation. Western cultural circles are always destroyed, such as Greek and Roman civilizations, which are completely destroyed, and countries and nations are also fighting to the death. The current federalism is the result of the balance of the law of the jungle thinking, mutual distrust, mutual supervision and mutual independence. Compared with the traditional system in China, you have me and I have you. Let China people say I don't trust you, so I want to supervise you. It is out of the question. Through historical development, the eastern and western cultures have their own distinct advantages, but their respective defects are also quite distinct. Now more and more people are trying to realize the mutual integration of eastern and western civilizations. Because of different civilizations, China's ethnic groups are better integrated than the West, and its historical traditions are more unified. Therefore, on the basis of culture and economy, China is not suitable for federalism, which will only make China gradually separate and weaken, and turn from a big country into a younger brother.