I don't think Orc and Other races are stupid, the the just don't care much about hiding their true nature. Think about it, There is no setting were humans don't make senseless war, political chaos and world ending magic or weaponry. Violent orc are the reflection of blood thirsty generals, and crafty goblins the reflection of every weaselly con artist. I think you mean do they "Dress and act like humans". Any race that can talk, manufacture, farm, and wage a successful war is intelligent.