Because the point isn't to give people more free time; it's to let people go home when they run out of mental energy and wouldn't be doing anything useful for the rest of their workday anyway.
Even if you were only working one day a week, you wouldn't be able to do 8 hours of contiguous useful information-work.
With 30 hours per week I would really prefer to work for 10 hours 3 days. This way after each work day I will have a day off. This way I can recharge myself sufficiently to have productive 10 hours.
As for productivity then I noticed when debugging hard bugs times really fly.
Even if you were only working one day a week, you wouldn't be able to do 8 hours of contiguous useful information-work.