Why do musicians borrow (from other musicians)?

For many, linking one of these four words - composer, creator, author, songwriter - to a musician might be an enormous leap of faith, and an assignment of talent, creativity and intelligence that all musicians do not, by default, possess. I disagree and feel that a performer has to add original expression to every musical performance especially if the music being performed is notated. This is because notation (in every system from every country I have studied) always requires at the very least a modicum of interpretation, and interpretation requires intentional creating, composing or authoring.