It's true that no one does 8 hours of coding. And you're the the last 8 marginal hours of development aren't as personally productive as the average hour. But personal productivity isn't organization productivity.
It's expensive to hire someone, its expensive to manage someone, it's expensive to keep someone informed about a project through meetings/documentation etc, it's expensive to rent office space for someone, it's expensive to have to hire more people to get the same productivity.
And all of these expenses are weighed against the reduced productivity of those extra two hours, and at 99% of companies they've decided it's better to hire for 40 hours.
