Uhmm.. mostly because the value an employee(CEO or not) brings to a business is already a probability function with variables(one of them simply being time and it's impact a unknown) and different expected values. If we believe that result-oriented rewarding is practiced, it's questionable that it can be estimated how much of an impact CEO can have and how it can be 200x of other employee. Infact, the reverse question of why pay a CEO a high salary should be the more common question, rather than why not.

-- I am skipping unknown unknowns for now, but you use your imagination.