The intensity of farm work isn't even comparable. You aren't toiling in the fields doing the same exact thing for hours every day, every month, every year like you do in a factory. Most labor happens during harvest and planting seasons, maintenance labor throughout the year is focused on improving your living conditions or other local projects, something you can't do if you are working for the factory. Unfortunately, things come up that can only be fixed with immediate or continued access to capital, so you have to send one of your family members hundreds of miles away to work in a factory to fuel an emergency fund. The problem with farm work isn't the work, it's the lack of enough pay to deal with disaster.
Because they've been forced to relocate from their farms to manufactured cities by the government which then bulldozed their villages.
Then once they're in the city, their choice is to work in the greenhouses of a big government-backed company, or work in the factory of a big government-backed company.
There are many documentaries about this.
Then why do Chinese farmers line up outside the gates of factories hundreds of miles from home in the hope of getting a job? If farming gave them an equivalent or better quality of life, they would just stay at home and keep farming.