This assumes that productivity is constant per hour worked, which is far from the truth for many professions. For most professions you the productivity delta not only decreases with too many hours, it even eventually becomes negative. There's no reason to suspect that eight is the optimal number of hours for every profession; it's just a historical quirk left over from the days of predominantly manual factory labor.
It isn't, but productivity also isn't an inverse function of number of hours worked.
At best, productivity is a function of the incentives which drives you, to a limit, after which it comes down for the number of hours worked.
Take for example there are many people who had high paying corporate job, but once they quit it and started working on their own startup, despite the long hours they worked, their productivity went up.
Similarly, other people work for money, because that's what drives them.
The main point is that there is no one size fits all solution. But nearly all these studies about workplace hours studies fail to see that.
Even in white collar that's not true. For creative problem solving it mostly holds but it doesn't hold very well in situations where the immediate product that isn't cutting edge and/or isn't mostly aesthetic (basically the artsy people and the R&D people). For everything else there's a pretty direct relationship between time and productivity.
People aren't machines.
They found production was maximized with an 8 hour day, interspersed with occasional longer days.
6 hour days would be a huge boon to my personal life, but I couldn't afford to be paid 25% less. even if I was 10% more productive (probably generous), i still couldnt afford to be paid 17.5% less