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 are plenty of opportunities to be busy without being productive. I'd say 30% of the work I see in offices is at best useless, at worst harmful.
So we have more office jobs, which have less hours, during which we work less, said hours being worth less.
And this is very uneven, because a nurse probably works more now than before. But all in all, office jobs are talking over in rich countries.