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.