Why don't the shareholders fire these guys out of simple self-interest to protect the value of the stock they're holding?

Because malignity is obvious in hindsight, not so much though when you're chasing short term profits and looking at various incentive schemes (bonuses) to make people perform. Which is what they end up doing, in a narrow sighted kind of way, focusing on fulfilling the bonus criteria by gaming the system rather than protecting the interests of their shareholders. Incentives of this kind lead to inward rather than outward focus; Barry Schwartz and Kenneth Sharpe discuss this matter at length in "Practical Wisdom".