Because it can be. The boards that set it are made up of executives, and it's good for all that the bar for average pay gets ratcheted up.
While a good CEO may be worth a multiple of any line worker, the notion that a CEO will leave for some other pursuit only holds water if the other pursuits hold higher multiples and are readily attained. They would all work for one-quarter the comp if that is all they were offered. As it is, they'll happily work for double also.
CEO's have a lot of influence on the value of firms they manage, as we're surely all aware of. CEO compensation is generally about aligning interests between the CEO and the firm itself.
Because just like congress, they can vote to give themselves raise and the amount. Can you as a typical employee guarantee yourself a wage increase, and the amount?
Not hard to see your pay increase as much as you want it to, when you're actually in control of setting it and not just accepting what is given.
While a good CEO may be worth a multiple of any line worker, the notion that a CEO will leave for some other pursuit only holds water if the other pursuits hold higher multiples and are readily attained. They would all work for one-quarter the comp if that is all they were offered. As it is, they'll happily work for double also.