I think part of the problem, is if the board fires the CEO for doing a bad job, that suggests the board did a bad job in hiring them in the first place. So it is in the personal self-interest of board members to try to play down the CEO's mistakes, to save their own face.
Whereas, a lower-level manager – if you hire 10 people to work for you, and one of them turns out to be a bad hire, people may think you still made a good decision with the other 9. Whereas, the board only really has one hiring decision to make – the CEO – so if they make a bad decision there, they don't have anyone else to point to.
For smaller firms and startups this is less of an issue, because even if the CEO turns out to be a disaster, likely few will ever hear of it–so it has less potential negative impact on board member's individual reputations. Whereas, for megacorps, if a CEO makes a mess, everybody hears about it, and board members are more likely to be asked about it when being considered for future board positions.
Whereas, a lower-level manager – if you hire 10 people to work for you, and one of them turns out to be a bad hire, people may think you still made a good decision with the other 9. Whereas, the board only really has one hiring decision to make – the CEO – so if they make a bad decision there, they don't have anyone else to point to.
For smaller firms and startups this is less of an issue, because even if the CEO turns out to be a disaster, likely few will ever hear of it–so it has less potential negative impact on board member's individual reputations. Whereas, for megacorps, if a CEO makes a mess, everybody hears about it, and board members are more likely to be asked about it when being considered for future board positions.