There's always a limit on how much time you can spend per day, and this limit is far below the time you are awake. Push it too far, and you do not only lose your working power, but also your motivation and your inspiration.
I agree entrepreneurs can get burnt out, but at that point it's become a "job" and is probably time to move on.
there are limits to stress your body can sustain, and your mind can be totally disconnected with it, pushing and pushing.
The assumption is not only incorrect, but I believe it is exactly reverse: Losing passing is one of the consequences of a burn-out. It is not a cause, but instead one of the (most visible) symptoms.