You'd be surprised how long poor CEOs can last. I worked for one large company where the market cap went to a 1/10th of what the company has spent on acquisitions alone during the CEO's reign and a continuous sequence of questionable business decisions that included laying off pretty much anyone who could have been instrumental in helping the company recover. I think there are many other examples.
A CEO can always blame the economy, blame the market, blame the weather, blame their predecessors or otherwise spin things in a favorable way. They can sell a story of tough times needed to make a turnaround. Sometimes there will be some secret scratching behind the back (I'm on your board, you're on my board). There's the cognitive dissonance on the board side of things, we've selected this guy therefore he must be good. etc.
As others have said, the incentives are set in a way that a CEO looks primarly after his own interests. It's safe to assume that CEOs that make a lot of money are even better at looking after their own interests. They are also very good at convincing others that they're not looking after their own interests.
A CEO can always blame the economy, blame the market, blame the weather, blame their predecessors or otherwise spin things in a favorable way. They can sell a story of tough times needed to make a turnaround. Sometimes there will be some secret scratching behind the back (I'm on your board, you're on my board). There's the cognitive dissonance on the board side of things, we've selected this guy therefore he must be good. etc.
As others have said, the incentives are set in a way that a CEO looks primarly after his own interests. It's safe to assume that CEOs that make a lot of money are even better at looking after their own interests. They are also very good at convincing others that they're not looking after their own interests.