You're right in that being unethical is the best way to get ahead in the game, and it might be a good idea to make it so that's not the case.
That said, the US of A is an adversarial society.
It's possible for specific people inside a corporation to behave in unethical ways. However, the definition of maximizing shareholder value is not simply "make the stock price as high as possible in the short term." It's perfectly possible for executives in a corporation to take the high road as the optimal strategy for maximizing shareholder value.
Thankfully, ethics do not depend on the existence of a soul nor conscience. There is a such thing as emergent behavior, after all.
Not if the shareholders want them to be ethical more than profitable. Arguably, shared ownership tends to promote turning a blind eye to that aspect.
Patagonia changed their corporate structure to a California B corp to allow this.