I really think it is connected to the fact that many managers never had to think about how long something takes and how much work they actually need out of someone. It used to be, that if you stare at your screen from 9-5 you probably did all the work you could. But now they can't see you stare at the screen and they fear they don't get all the work out of you. For the first time management has to figure out how much work they need instead of going by time spent in the office. That scares them.