[username90]

The maths behind long workdays is pretty simple, you have as you say around 4 hours of productive hours per day, so why do companies want you to spend 8 hours in the office? Because the more free time you have the likelier it is that you will spend those productive hours at home doing hobbies! Therefore the goal of long work hours is to prevent you from having a life which could get in the way of work.

Or from another angle, a person who voluntarily works 45 hour weeks likely spends most of his productive hours at the office. A person who do 35 hour weeks likely spends most of his productive hours at home. A person who is forced to work 80 hour weeks will definitely spend most of his productive hours at the office since he has no time left for anything else. It might hurt his overall productivity, but it at least guarantees that the company gets his all.