The second reason is that after the urban reform in China, especially after the establishment of special economic zones, the huge human resources have attracted farmers who used to be in the fields to factories and cities.
The third reason is that the rural population base in China is too large. China has too little arable land per capita. Surplus laborers must go to cities to find better living space.
Fourthly, before 2000, China's rural policy was actually unreasonable. Farmers are under much greater pressure than urban residents, so it is better to farm for half a year. At this time, a lot of land was abandoned and they all went out to work.
The fifth reason is that China's social development and public facilities construction are imperfect, and urban residents often enjoy these public resources. In addition, it is difficult for rural people to enjoy even basic medical care and education. This is why farmers in China have always wanted to go to the city.