Nobody said anything about paying less... In my mind if I were to run a company I’d do 6 hour work days with four day work weeks. Nobody actually gets a solid 8 hours of coding done. I don’t see why people keep throwing money at more perks/food to compete with FAANG at their own game.
It's true that no one does 8 hours of coding. And you're the the last 8 marginal hours of development aren't as personally productive as the average hour. But personal productivity isn't organization productivity.
It's expensive to hire someone, its expensive to manage someone, it's expensive to keep someone informed about a project through meetings/documentation etc, it's expensive to rent office space for someone, it's expensive to have to hire more people to get the same productivity.
And all of these expenses are weighed against the reduced productivity of those extra two hours, and at 99% of companies they've decided it's better to hire for 40 hours.
What if the studies that say extra time in the office is often wasted are true? Should people give up some comp to not sit in a seat?
The studies may be wrong, or maybe you pencil it out and it just doesn't model out correctly, but if you lock in the pay on the basis of the assumption that the total productivity would be less, it's sort of favors one side of interpretation. And if you're truly trying use a four day week to attract high-end talent, reduced per hour pay somewhat dilutes the attraction of the benefit.
Finally, some consulting gigs are billed per hour anyway, so the model might end up that total comp is somewhat reduced vs a full 40/hr week. Or it might end up the same pay for four days - it really depends on the business model.