Earlier quoted context omitted.
That's literally missing the entire point of it. You already have this now - always had it. There's nothing new or interesting about what you're "proposing". What IS new and interesting is getting employees who work 20% less, AND THEREFORE are 20% more productive (and presumably 20% happier), at the same cost.
Wow... So like magic? Why aren't all successful businesses already doing this if they can get more productivity for less time?
40 hours isn't some magical "right" number. It was something that was decided before computers, Internet, global fast communication, etc etc etc.
I think its beyond late for us to reconsider those hours and decrease them.
To answer your question directly: Why would they change what's been "working" for them for decades? The only way we can see big companies change this is if a startup gets really successful and just has this as the norm. That'd create market pressure for change.
Where that market pressure doesn't exist, regulations can play the same role.