Good question. The short and unsatisfactory answer is because "that's the way it's done", and any professor who decided to copy someone else's course would face

Good question. The short and unsatisfactory answer is because "that's the way it's done", and any professor who decided to copy someone else's course would face heavy criticism, and neither the students nor the department would feel like they'd be getting their money's worth. The system could work a different way, but it doesn't, and you can't really get there from here.