School is about a person who knows more than you on a topic, being over your shoulder, and telling you "that is right", "that is wrong", "you should do this via this method", etc.

School isn't just a funnel of knowledge being poured into your brain. It's about the people that teach you and verify that you are actually learning the right things. Sure, you can pick up books on cryptography, but who would you rather hire? A self-taught cryptographer, or a university-vetted cryptographer?