If burnout was a natural byproduct of working too much (or too hard, or for too many hours), we’d see every founder/C-level suffering from it.

Oh, a comedy piece! Well deserved sarcasm aside, that bit only tends to be true to small and maybe medium companies, where the top isn't an impenetrable ivory tower.

the real energy drain is when they ask themselves: Why am I doing this?

Because they have to eat and pay their bills, that is literally the only thing keeping people doing the soul crushing shit thrown at them.