Because we need to rely on something, unless chaos shall rule. Perfect it never can be, I'm sure, hence we rely on heuristics. And I am not referring to science here. :-)
Without the opposites, we don't know right is right, wrong is wrong. Good question - implication of this question reminding us need to see everything / every event / every argument which opposite state it is in, any the other opposite state we missed out before making a "balanced" / "informed" decision.
First, life is full of opposites; good and evil, justice and injustice…, etc.
Similarly, we, out of logic, need to classify matters around us (based on our set of beliefs) to what is acceptable and what is not. As we need some sort of agreement about how our lives should be. Regards.
All through history, there has been a moral code which resulted in social classification of attitudes & deeds into right & wrong with a majority consensus. An individual draws his/her view of right & wrong from this background. It is very rare to find persons turning a social judgment upside down unless they are revolutionary.