Learning and assessment

We never want to get assessed until or unless we have that confidence that we knew everything.I never had any encounter with a person who loved exams.Whenever anyone has to give exams they have some fear  and some discomfort related to it.This is because of the various flaws in our examining  process. How can a person can access a learning of 1 year in 2 hrs.Why assessment is done by evaluating what the examiner want to know not by evaluating what the examine know?