I can believe that people normally can't get as much done in the second 4 hours of their 8 hour day as they do in the first 4 hours.

I can't believe that people can get as much done in 4 hours as they can in 8 hours.

Less output == less revenues == ultimately less pay. You can't change math, companies aren't going to get paid the same amount for making less stuff or providing less services.

Maybe the difference is only 30-40% less instead of 50%, but I think most people are willing to put in the extra 4 hours for the additional benefits.