Why do people in their 20s attempt to brag about their scientific or professional skills/achievements?

Son, you are literally a beginner. Just focus on learning and being humble. The earlier you realize how little you know, and how much more your superiors know, the better position you are in.

Just had this guy in a work interview for me claim that he has basically mastered mathematics. Btw, for an engineering position. I tried to hide my pity for him.