Because copyright is a human invention designed to encourage creative work by protecting what is otherwise easily copied and transferred around. That copyright protects the creator from unauthorized reproduction of his creation for a long enough period of time to allow him to recapture his investment. Once that period of time ends, he no longer has such protections and whether he likes it or not, his goods become public domain. He has plenty of time to both enjoy profit from his creation, and time to create something new which could subsequently be protected for another period of time.
It was never intended to grant a creator indefinite immunity from reproduction of his creation. Very few individuals would either a) care to have their tax money spent defending greed, or b) be held hostage to someone who wants to rest on his laurels and have his publicly granted copyright used to allow him to become useless to society indefinitely. What we giveth as a society, we also taketh away, copyrights and patents are two such things.