There are two main arguments, the first is that, as mentioned, you have to pay to get the best, and at the top end, a CEO who generates 1% more profit through better leadership might be worth 10x more than the lesser CEO.
The second is that most executive positions also involve an agreement to waive many of the employment rights we take for granted, especially around dismissal and so on, in the form of severance agreements. If Fred Goodwin had been a standard employee, they would not have been able to dismiss him for the issues the company was facing, because everything was done in good faith, in full knowledge and with the consent of the board. Under his agreed contract terms, however, they could request he leaves. That sort of contract tends to involve lots of 'make up' to ensure agreement.