Man, I remember when I lived in China and we would have the wifes of our engineers sitting out in the stairwell crying because the husbands worked so much and were never home. They certainly did work a lot. Some of them I could never beat at being early in the office and leaving late and I find myself quite good at that I would say.
IMHO (French, expat in Shanghai since 201708) in the Chinese middle class (and up) parents usually push their children very hard. Many start their day early, study before going to school, after it, and even during weekends. The school program is way more ambitious than most (maybe even all?) its occidental counterparts. All this is very competitive, even on extra-scholar activities (art, sport...).
Later, as adult, most are used to the idea that they must work hard, to the point of not even considering another approach.
RIP Zhang. A change in 996 would spare China's young talent, reduce brain drain, and maybe lure educated people from abroad to help Chinese companies expand into new markets. I hope something can change, stories like this are heartbreaking, though not unique.
Just a general question, is it known why overwork and things like playing games for long hours has not just benign effects like fatigue, but actually leads to sudden death? There is the notion of karoshi in Japan which is death due to overwork. I also remember some news articles about a gamer in South Korea who collapsed and died after 4 days of continuous gameplay.
I only have anecdotal answers, but a few of the South Korean and Chinese video-game related deaths were caused by cardiac arrest and dehydration. I always assumed the unspoken part was that the gamers were using some mild stimulants (at least large doses of caffeine), combined with long bouts of inactivity.
This became a somewhat related issue in Japan a few years back, sometimes tied to karoshi incidents, and led to very strict regulation of stimulants, including how energy drinks and caffeine pills can be packaged / sold.
There are many other examples of cardiac arrest after several days of low activity and sleep deprivation though, so the medical reasoning may be simpler.
Modern China has a pretty short history and many in power today went through a more difficult time when they had to fight for their survival. 996 might seem totally reasonable to them. There are also many role models (i.e. https://en.wikipedia.org/wiki/Lei_Feng) who sacrificed their health/life for work being praised in literatures.
Also, China is not a wealthy country, slipping into poverty is still a real possibility for many.
