A question about football manager's contracts

It is a fact that at any one time there are more unemployed managers than there are vacancies, add in the qualified potential managers awaiting their opportunity and the difference between vacancies and potential employees is even greater. With that in mind the power evidently lies with the hiring club so why do clubs such as ours allow untried and unproven managers to to dictate that they get a huge sum of money if they do not perform in their job? There must be a reason but I can't think what it is.