Okay, so when we do book reports etc. I did get why would couldn't write a report on a book we've already read, but never did a report on. Why do they make us do that? I find it quite annoying
Because they want to keep you reading. That's why. They have to. It's their job.
But relax. Almost everyone hates the books that they have to read for school. But it also makes you learn stuff. You may never actually need that stuff ever again, but when you are grown up, then it makes you sound cool in conversations.
because they want you to be exposed to different styles of writing, specific authors or genres... its good for you little undeveloped over internet and tv exposed brains you should thank them :)