Except they can't, which is why other people are hired to fulfill those roles. I can extend your argument and say that the CEO should be involved in every detail of the company because, after all, "if the CEO doesn't have time to make sure the company [is doing X properly], hey don't have time to be the CEO."
The CEO in the linked piece didn't need to get down into the weeds of the accounting laws, he just needs to know to get a second (outside/unbiased) opinion on anything that smells even faintly and remotely fishy that he's signing off on, and he needs to keep a general eye on principal/agent problems or conflicts of interest in the people he manages.
And he needs to know that the outside opinion is actually the same opinion that would be shared by a potential prosecutor / judge / jury. And he needs to have enough intuition to know that something smells fishy. And that it is a bad fishy smell, not a good fishy smell.
