The reason the golden parachutes exist is because CEO is likely going to be 'the end of the career' for most. There are so many reasons to get fired and it will probably happen. After that, there's not much else.
Nobody in their right mind would be a 'Big Co CEO' without some kinds of rewards even on the downside - many of these folks can make quite a lot of money in some other C-level position and do just fine right there. Why put your head on the chopping block?
You might also ask the question why athletes are paid gazillions for doing something fun, that they enjoy that comes with zero responsibility basically, and can often do very well even if they don't perform, although usually that's not the case.
And FYI most CEO's don't make gazillions of dollars, that's just what we see in the news.
on example was the campaign against a former BT CFO would not would print shares to go on a buying spree - like Vodaphone was doing
Yes, a "bad" CEO will be fired, meaning that he will pocket a fat severance package and he will be able to try again in another company with almost no stigma attached.
Even if he did it so badly making himself unhireable for similar positions, he will have already earned more money that most people (even with professional jobs) see in their lifetimes.
"With great power comes great responsibility" seems not to apply to CEOs, more like "the house always wins".