By that logic, why bother with democracy and not trial by combat?

The problem with your logic is that you assume jurors don’t have a sense of ethics and justice. If they truly don’t, then forget the judiciary as a problem, because the society itself isn’t going to hold up. So in that way, applying your logic here and under that assumption you are right, why bother with democracy and not trial by combat when people no longer care about acting in good will?