The argument (and I’m not saying it is or isn’t backed by sufficient evidence because I don’t know) is that the lost 20% is because we are physiologically not capable of retaining mental focus for 8 hours a day for 5 days in a row. There is some evidence for an equivalent claim for physical labour, which is part of the reason we do 40 hour weeks rather than 70, though again I don’t know how good the evidence really is.

To put it another way, think of this as the mental equivalent to why we can’t run marathons at the pace we can do a 100m sprint.