Why did a large number of people move south in ancient China?

There was a famous southward movement in the history of China. At the end of the Western Jin Dynasty, five people made a fool of themselves and crossed the south. The north is in a pot of porridge. If you don't go south, you will die. One was at the end of the Northern Song Dynasty, the shame of Jingkang, and the Jin people plundered the south. If they don't run, they may die.

Otherwise, it is difficult to migrate in large numbers because of the feelings of ancient people in China who are unwilling to leave their hometown.