#1 Quitting does not mean you never look for another job or quit on the first day etc. It means when you perceive working for said factory is not worth it you do something else.

#2 Having a large labor supply suggests the job is competitive. However, even if that's the case having people constantly quit increases training costs.

The point of all of this is industrialization tends to follow a path where people move from terrible low wage jobs to more well paid work. Even China has moved past the first rung of the ladder with market forces forcing factory's to increase pay and or improve conditions. Presumably where going to run out of 3rd world countries at some point, though there is some concern robots / AI is going to end up filling that nitch possibly stranding a few countries.