Right, but that might be because we’re all more poor.
If I produce $1/hour of profit (for myself) and I work eight hours a day, then I’m making $8/day. If I double my productivity, I can work the same amount of hours and earn $16/day or I can halve my work hours and make the same amount as before (or I can do something else).
The question of how many hours to work will now depend on the relative values of my free time and my work time. Each hour I work is now worth $2 to me, but it costs the same as before in terms of effort. So I should be willing to work more by that calculation. However, IIRC and I don’t have a cite for this off-hand, empirical studies have found that people are fairly consistent in valuing their free time as being worth about half their wages. So if I’m making $1/hour and increase it to $2/hour, does the value of my free time increase from $0.50/hour to $1/hour? Maybe, as my income increases the range of activities I can engage in increases, and presumably the value of them as well. Before I could watch my black-and-white television and now I can watch my color television. Before I had to go to the library, now I can go to the movies. Before I had to eat plain white rice, now I can eat at the Ritz.
So to answer why we keep working the same hours despite increasing productivity, we need to figure out how these values interact and change over time.
But first we should ask ourselves if we really are working the same number of hours. I just went to the Bureau of Labor and Statistics and since I can’t link to the output results, follow these steps. Go to the most requested series page. Check the box labeled “Average Weekly Hours (Unadjusted) - EEU00500005” and click Retrieve Data. Under Change Output Options, use the pull down menu to start the data in the '40s, let’s say 1947. Be sure to click the box labeled “Include Graphs NEW!” and then click go. Now look at the graph. It looks like hours fell pretty steadily from '64 to '84 (damn you, Reagan!) and have held steady since then. So we have cut back, but not a lot. I suspect it’s because our free time options have improved over that period as well, but I have no proof.
Finally, you have to consider institutional options. Basically, you can’t get ahead in life without working 40 hours or more on your job. The Man demands forty hours out of us. Personally, I’d like it if some enlightened legislators lowered full employment from 40 hours per week to 30. Forty hours may be an equilibrium because it is difficult to change to another number of hours. So social inertia may have something to do with why we keep the 40 hour week.
Summing up this long-winded non-answer, I think that it is a suprisingly hard question to answer. I don’t know, but I hope I’ve pointed a path that may help.