But then you're expecting all CEO's to be 10x and are making a hypothesis that many are probably not. Maybe.
Look at it from the point of the Board. They don't care about the details. They just want to appoint people who will handle it. They don't have the time to supervise every company they're the board members of (possibly a problem of corporate governance to be sure).
So CEO's tend to be the folks who: 1. Get things done and 2. Accept responsibility
As long as they execute on these goals, the Board doesn't care. In fact, they will pay the CEO more if they take such good care of the company that they don't need to be involved at all.