Any employees that really create much more value than their salary can and will just leave and start their own company, where they get to set their own salary. By doing so, they'll also be leaving the pool of employees, and potentially hiring others too (reducing supply, increasing supply, driving salaries up).
Of course, every employee creates more value than their salary, to cover overheads, some profit for shareholders, etc. Employees that claim to be creating far more value than their salary, but not leaving to start a company, are probably deluding themselves, and vastly underestimating the value of the capital employed in their company, goodwill with customers, the sales and management functions, etc.
