Why do we have exams?

I'm really starting to take in that exams are pointless. If I've been taking a course and I've been tested every step of the way, then why should I have a test at the end on everything that I have already been tested on? Exams are usually worth 20%-30% of your final mark and I think that's unfair. The majority of people end up lowering their mark after exams and it's stupid. If I have a 80% in a class, it's pretty hard to get that high on an exam so most likely after the exam I will not have that 80 any more. On top of that, exams are really stressful, since we're all pretty busy we don't always have time to start studying for them far in advance and most of our studying ends up being a night or two before the test. I think exams should be terminated or at least be removed from subjects that don't require an exam. In grade 12 fitness you only spend a week doing health to prepare for the exam and basically the whole semester is working on your personal fitness, so why should we have an exam in a class like that? It makes no sense. No matter what subject it is, I still hate exams. Sitting down for 2 hours straight to write a test for either math, science, English etc. is annoying. Not to mention the fact that I have to wake up early just to go to school and write a test that is a an hour or two, its honestly ridiculous. I'm honestly tired of school and can't wait till I'm done.