Many workers come from the farm areas and treat it as seasonal work. This is why production from China is often slow after the Lunar New Year, many factory workers simply don't return after the holiday. Anyway, the pay is only average (and very low by Western standards) but it's still considerably more than what they would be making in the rural areas. In the US, it's comparable to working on fishing boats in Alaska in that you can make a relatively large amount of money in a short amount of time but you probably won't do it for more than a season or two if it isn't your life's work.
I've been to the factories that make Fender and Harley Benton products. They're a bit dreary but are well ventilated and there's nothing I regarded as safety concerns. The Chinese government does regulate working hours and I never saw anyone look like they were being worked to death. Ningbo reminds me of a turn of the century American Industrial Revolution factory town.