Suppose you're a factory owner with 4 workers working 40 hours per week. You invest in a new machine that lets everyone be twice as productive. If you don't have demand for the extra output, then do you cut everyone's hours in half or let two people go?
Think the far more common solution is to let two people go. So even though productivity went up, hours worked per week stayed constant.
And I think 40 hours per week is status quo. Suppose you get a job offer, and ask to work just 30 hours per week. There are a number of forces conspiring against you: 1) If you're salaried, the employer would see this as less efficient (even if you're actually more efficient) 2) If you're used to a particular way of living, 30 hours could entail pay cut 3) Others are willing to work 40 hours per week, and hence employer gets more "value" out of hiring someone else per week
I think best bet to achieve that reality is to be your own boss. Friend of mine works as a UI consultant and bills out at $200 per hour, works maybe 20-30 hours per week. It's a brilliant gig.