Cause 40hrs a week is a schedule for workers on a production line with machine tools doing monotonous work. It's hard, but it doesn't require you to think much. Thinking, changing contexts is hard.
Ah, also you really are a resource, only your employer is a resource for you too, to get money which you then use for your own purposes. You are mutually resources for each other, that's the point.
Well, also it seems that in the olden days, when we didn't have internet etc, it was a bit more normal to do your own hobbies etc at work, unofficial tea breaks, and in general many things other than work. Though I'm from Russia, and the Soviet joke says "they imitate pay, we imitate work".