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.
If only some companies make the switch, you could bring an argument from 'efficiency wages': ie by offering a perk to your workers that they can't (currently) get elsewhere, you make them work harder and avoid wasting time, because the outside options are worse so they want to impress to keep their cushy job secure.
I don't know whether that's true, but it's a plausible argument for why people would do less of the unproductive things you mentioned.
If that's so easy, why not stop doing those things and keep the 5-day week?