A significant chunk of human knowledge is not publicly accessible. You cannot self-teach how to make a modern aircraft, jet engine, nuclear reactor, radar tech, advanced metallurgy etc.
Similarly, I would wager most of the useful economics and financial theory that humans have come up with is only known to hedge or prop trading firms.
For some subjects, the entire journal-published academic body of knowledge for it is probably some useless fraction of the whole and university academia is operating nowhere close to the cutting edge. People are probably doing PhDs today on theses that some defense contractor or HFT firm already discovered 20 years ago.
Even things like specialized medical knowledge, I would wager is largely passed down through mentor-mentee tradition and/or private notes as opposed to textbooks. It's unlikely that you can teach yourself how to do surgery just from textbooks. I once had a pathologist's report use a term for a skin condition that was quite literally ungoogleable. The skin condition itself was fairly ordinary, but the term used was outright esoteric and yet probably used on a daily basis by that pathologist. Where did he learn it from?
Not everything is on the Internet.