38 hours (the estimated work week) is exactly half of the 996 week (76 hours) of some companies in China. I don’t think that’s healthy, but if I had to bet, I’d pick the 76 hour workers outcompeting their 38 hour counterparts.
For highly-educated jobs where people need 6 months of onboarding and institutional knowledge matters, it's probably more efficient to reduce churn.
That would be lovely. Unfortunately, many places would rather you move job than give you a raise. It's cheaper in their accounts, and that's what (ah) counts.
This is consistent with many studies showing that above a certain level of revenues, money matters less and less, while free time matters more and more.
Since training is expensive, what companies are looking for are people that are willing to work more so that those costs are divided amongst more working hours. And they notice that paying more does not translate in people working more, for the reasons outlined above. A equilibrium is reached when people work the amount that they need to reach a certain comfort level then switch to looking for more free time.
And to be sure, there is no one-size-fits-all answer. What the number is probably depends on the task at hand--though I would argue that (all other things being equal) it's probably closer to 0 than 18 for any job.