The only reason things are done that way is because it's the only way to do it. You can't grade a person without comparing their performance to others, unstandardised exams can be the subject of favouritism and unfairness.
Also if not everyone sits the same exam then the exam difficulty can vary making it unfair for all candidates.
You'll all realise the importance of this method of testing when you grow older. Getting good grades requires hard work, period.