1. They are somehow superhumanly increasing the company's bottom line
2. They target companies with enormous amounts of money and intentionally influence their salary to become very large, independent of their job performance
Insofar as #2 is possible, that seems the most plausible explanation for large CEO salaries. Unless the people determining the CEO's salary are somehow completely firewalled off from his influence, it is hard to see how #2 is not possible.