That's why there is the principle of stare decisis, which says that deference should be given to previous decisions. Otherwise judges could change their interpretation for each case which would mean there would be no law at all, just what the judges want.
Also, stability of the law is not to say that the law should never change, but it should be determined by the people and their representatives, not unelected judges. But Congress doesn't act to update the laws and they just leave it up to the courts, which erodes democracy. It's been over fifty years since we've had a new amendment to the Constitution.