Regardless of whether we actually follow the moral law, we expect other people to follow it. We feel indignant and full of righteous anger when other people wrong us. Our reactions to their behavior betray that we actually do know the moral law exists (and we take it seriously). Even when we disobey the moral law, we uphold it as the standard by which others should treat us.
The fact that everyone breaks the moral law doesn’t mean that it’s nonexistent, or that it doesn’t matter. The moral law is still the standard we think ought to be upheld—at least by everyone else!