Copyright is a deal with society. One might just as well ask why an author has any say over transactions between two independent third parties. The answer is be

Copyright is a deal with society. One might just as well ask why an author has any say over transactions between two independent third parties. The answer is because it has been deemed useful for the encouragement of creative works to provide for that say. It's not immutable and the rules can easily be changed if society thinks they should be.