I think a big factor for this idea not catching on is the grey area of what qualifies as 40h workweek labor. Some jobs like customer service positions make sense, but what about jobs that require a 24h standby? What about positions with fluctuating periods of downtime through the year? what about jobs that could be 8h workdays, but they haven't yet been confirmed that they could in fact be more effective as a 6h or 4h workday?
It's a by-product of modern times not quite catching up with modern technology. We certainly have the ability to think up algorithms that would better suit certain positions based on these factors, but the idea of it right now is too drastic and 'unfair' sounding. Ideally, in time, with the right evidence proving it's effectiveness, workdays would be shortened, quality of lives would bump up and robots would gradually take over.