What is the reason why the middle and lower classes in China are bent on immigrating to the United States?

There are many reasons! The first is high wages. The more people at the bottom of China society go to the United States, the higher their living standard will be. China's middle class may not improve their living standards significantly when they go to the United States, and they may even feel cheated.

Besides, the West is a world where the fittest survive. The bottom poor who can't get along at home will only be poorer when they go to the United States. Poor people who have no ability are poor wherever they go and rich wherever they go.