Are Written Examinations Justified?

Finally my month long exams have ended, and I'm left wondering. I'm given 3 hours to write so many answers which is not totally based on my knowledge but mostly on my memory.

To a question I have written all the points relevant to it, not going around with the active and passive voice thing to make the answer look bigger. But I don't get marks for answers which are to the point. Teachers tell me that I didn't write enough, and after showing them the answer from my textbook, still they say there's something missing. The question paper may specify that the question has maximum marks of 6 or 7 marks but the ones who check your paper that they haven't given anyone above 5.

I have never been able to solve this mystery about what do the moderators actually want from an answer. The stupidest incident I had was when my teacher told me that if you haven't taken an extra supplement, you haven't written enough.

Writing in exams is too much to remember. I'm a slow writer and usually I'm not able to finish writing all the answers. So does this disability decide my future? Do the written exams really test your knowledge about a certain subject? According to me objective questions are the best since they test out your conceptual knowledge, but no one wants that. Really confused!