This is the superman myth, it doesn't exist.
If Bob, our intrepid entrepreneur, actually worked 40 hours a week he wouldn't spend so much time faffing. The 'busy work' disappears. Overall productivity drops the more you work long hours.
I've nowhere near your experience with entrepreneurs, I totally admit, but one of the symptoms I've noticed in crunch entrepreneurs is their mental diarrhoea, they take 15-20 minutes rambling on talking to you about a decision you've asked for that they normally would have made in a minute or 2. You start walking away and then suddenly they have another point/thought. They're mentally exhausted. I can reflect back on myself and see the same thing when I was doing crazy hours.
Some of these people have succeeded, some haven't.
But the person them self doesn't notice. They still think they are operating at high productivity.
I know people can work many more than 40 productive hours a week for years at a time because I did it myself during Viaweb. I know lots of YC founders who do as well.
I also know from my own experience that this ability varies between people. I can't work the hours now at 47 that I could at 30.
Why shouldn't ability to work vary among humans? Practically every other ability does.
As a result, they see high productivity, but do not see the focus shift. Staying on task is what is important in crunch mode, and there may be some utility to founders that have more tasks (and thus goals) in shifting focus, but larger organizations suffer from defocusing, task shifting, and worker collisions.