Laws are written and considered and passed by a relatively small group of people (only 51%+) in Congress. Those lawmakers are only put in power by a majority vote (and remember, not all people vote) and they, and the issues / people they represent, only stay in the Congress for a limited period of time.
I think we can all agree that they are not perfect and their ideas are often based on the politics of the moment.
Then why do Federal Laws last indefinitely?
Every law should have an expiration date. An important law, of course, could be brought to the floor, unchanged, re-voted upon and reinstated. But there are really ridiculous laws that persist on the books making all of us subject to a narrowing window of liberties and unnecessarily complicating the legal code.