Try thinking about it with the limit not being the hours per week on average you're at work, but the total number of hours per week on average that most people are capable of being fully productive.
In other words: If people at work 40 hours a week on average, but only have the mental stamina for 32 hours of work a week on average, why stay in the office 40 hours on average? Those 8 extra hours will not account for extra productivity. Switching to 5 day weeks, in this model, won't increase productivity, nor 6 day weeks.
I've said "on average" a lot, and that's very deliberate. Most people I've ever worked with across 20 years in tech are largely unable to keep up a fully-mentally-engaged 40 hour work week every week, and even fewer are able to sustain longer average weeks than that for very long at all.
