Replaceability.

You can find a lot of people who would work as janitors, as they lack the education to work a job that pays better and has better working conditions. But there are few people in the world who are experienced and knowledgeable enough to be the CEO of a large company, and willing to accept the huge responsibility that comes with it.

In general companies will want to keep wages as low as they can. But they can't pay their CEO candidate low, as he would just go to another company that pays better instead. He could even choose to not work at all if no company pays enough to justify the responsibility and work load he will take. So companies have to pay him high if they want him.