"... research shows that knowledge workers actually have fewer good hours in a day than manual laborers do — on average, about six hours, as opposed to eight."
Motivation to build what you really want to build is completely different from being motivated to work for a salary to avoid starvation and keep your family going.
I imagine it's a lot easier being a doctor who is actually helping individual people squeeze in one more patient, vs. spend overtime filling in paperwork. Same difference between working on an awesome product where you have total visibility into the whole process, vs. beng a cog in a much larger wheel.