Students will spend most of their time learning about what others have said. The subject is not a series of debating sessions in which students will develop their own theories. If this comes up, then you could refer to two good reasons for this. You'll be looking at hard questions that philosophers have thought about for a long time. You'll need to know what they thought to avoid 're-inventing the wheel' – to avoid spending time working out a line of thought that someone has had before.
Philosophical questions are tough questions. You wouldn't expect to do physics or maths and be asked to come up with your own theories. You need to learn what others have said to learn about the topic itself. It takes time to become an expert.