It's because the only people who can understand the code are people who write the code every day. And sometimes the devs don't even really understand it.
Occasionally you'll have a manager who manages in the day and codes at night, which is fine if not bad for his personal life. But HIS manager almost never does that. And his manager's manager? Forget it.
Occasionally you'll have a manager who manages in the day and codes at night, which is fine if not bad for his personal life. But HIS manager almost never does that. And his manager's manager? Forget it.