It has. 80 years ago your worked more hours, and you could not slack at work. Today, you work not only less, but half of the people in the offices I go to don't work their full shift. They go to facebook, send text messages, take long breaks, do their shopping and watch porno (yep), etc. Also, an hour of work doesn't have the same value. When your work is moving boxes, an hour is an hour. But in an office work, there…
If there is so much useless/wasted time at office jobs, what's the practical purpose of requiring them to be there for a fixed number of hours?
The other problem is that if you measured people purely by output, 10x developers (assuming you believe in such a concept) would be able to work 1/10th as much and would very visibly be missing much of the rest of the time. This would lead to a lot of resentment from people who are less productive and would be having it rubbed in their faces.
To get to a place where people are judged purely on output we would need to change society such that the notion of equality was essentially never mentioned or thought about at all. Because people's productivity is wildly, wildly unequal.