Sure, there are lots of reasons beyond Homo Economicus thinking for why we work the way we do. Laws, cultural expectations, and so on. I just don't see results-based work displacing the current norms broadly enough to make a difference, and if it does in fact result in better productivity I expect that to be eaten by competition/profit rather than going to workers as free time, in the long (medium, even) run, absent some pretty massive shifts in culture, especially with international competition in the mix.
9-5 isn't the 'optimal'. It's simply what is 'most common' for a diurnal society. Nobody is driving around town looking to get a hair cut at 3am and staring at an IDE from 1pm-5pm trying to solve a bug isn't making you any more efficient than thinking about the problem while cooking an early dinner.
Humans do suffer fatigue - both mental and physical - and proper lengths of rest helps both kinds.
I posit that happier employees are more likely to trend their work/life balance in favor of work if they enjoy their work and enjoy their company. Unhappy employees will skirt by on the bare minimum required knowing that hiring/training a new employee isn't worth the cost of firing them.