You missed a very important Hayekian point in your analysis: human beings will always be ignorant about the functioning of a complex society. The same for governments. Morality evolved through a process of social selection, and it's purpose is to coordinate the actions of members of society so that they can become predictable to the other members. Moral rules evolved as a consequence of the ignorance of human beings to most of the facts in their social environment.
You seem to be advocating a model of governance based on utilitarianism. You are assuming a priori that governments CAN make good decisions if they know the facts; however, they can never know all the relevant facts that can affect the consequences of their policies. Moral rules come into play here; they make decisions more predictable so that members of the society can adjust their actions accordingly. Therefore, it is in fact better for rulers to take decisions based on established moral rules, than relying on their- necessarily flawed- judgements of each particular situation.