> I don't understand why.

It's because you don't live in a highly competitive society. People try everything they can to squeeze extra points out of these exams. It's the only way to get into good universities. Why do you have to go to good universities? When thousands of people applying for one job, the company will try to find the fastest way to filter out applicants, and your degree is the easiest way for you to stand out. In an ultra competitive society, you do everything you could to stand out, otherwise you won't stand a chance.