I don’t have any hard evidence for this, but my intuition is that it’s about the trade off between the cost of hiring and training workers versus the work outpu

I don’t have any hard evidence for this, but my intuition is that it’s about the trade off between the cost of hiring and training workers versus the work output you get from each one versus the workers desire to earn income. For the employer, it would be better (in theory) to have fewer people working for more hours, because then training costs less and the workers become more and more expert. For the worker, the desire to earn more is most easily accomplished by working more. Both of those are offset by the decrease in productivity and increase in fatigue and burnout as you work too many hours. Right now, most workers and employers have landed on about 40 hours a week as a good balance between these concerns. However this has changed over time and will change in the future. For example there’s recently been an increase in so-called “30 hour jobs.” For example: https://30hourjobs.com/ https://30hourjobs.com/