The value of their contribution to the company's commercial success is much harder to assess. And, in any case, they're the ones in charge of the assessment. According to corporate law, the managers of a company are merely the servants of the company's owners, the shareholders. So if the managers are paying themselves too generously, the shareholders have the power to step in and correct matters. Obviously, the real world doesn't work that way.
Economists see the relationship between shareholders and managers as one of principal and agent.
In theory, the shareholders could pass a motion at the company's annual meeting cutting the CEO's salary or disallow a motion to grant him a large dollop of share options. In practice, such campaigns are rare - and their success is even rarer. An individual shareholder has too little to gain to justify the huge effort and expense involved. What's more, I will gain from the success of your efforts even if I take no part in them. So there's a great temptation for small shareholders not to bother joining such campaigns.