We as humans are not designed to be productive selves 5 days in a row, for 8 hours a day. If I'm not feeling well or having a bad day, I might finish working an hour or two early but make up for it by working overtime another day that week. It doesn't matter that I cut that day short - I wasn't going to be very productive if I wasn't feeling well anyway.
The 9-5, 8 hour work day was designed for PHYSICAL labor during the industrial revolution where output was directly correlated to time. This is not the case anymore. If someone can meet/exceed their responsibilities in 5 hours, why do they need to spend 8 hours just because slower workers need that time? This just punishes efficient workers.