Mostly risk. CEOs make certain decisions under lots of uncertainty and if they're successful they'll earn a lot if not they're replaced with someone else. It's rare for a dismissed CEO to be a CEO ever again.
This just feels wrong. If you gave those conditions to the job, nearly every person on Earth would take the job. Who cares if I can't be CEO again, I probably was never going to be one in the first place.
CEOs make a ton of money because they supposedly add a lot of value. Whether that is true or not seems debatable.
I think his salary is $250 million a year. Thats so high. But i think its worth it for Google.