Because two people need to do it. Back when it was called 40 hour work week, only men worked, so on an average, a couple worked 40 hour weeks. Now, a couple works 80 hours.
They are the ones that need couples to create families to produce children to feed their capitalist machine, so this is super relevant to them.
So, from now on, let's call it a 80 hour work week, because it sounds much more accurate than 40 hour work week. Companies measuring man-hours (however dumb the metric is) always multiply hours by manpower, and we should do the same.