People speed all the time, especially in high-traffic urban areas. Does that mean that speed limits don’t exist, or that they’re less real in big cities where more people speed? No; the fact that a law is broken doesn’t mean the law doesn’t exist. Law by definition tells us what we ought to do, not what we actually do. It’s prescriptive, not descriptive.
It’s true that people break the moral law all the time—they do things that everyone knows are wrong. But if all humans break moral law, that makes it all the more likely that moral law is something completely independent of human behavior. No matter what humans actually do, the moral law stands outside of and above them - and tugs on their hearts.