1. In my experience, there is a lot of introductory material to be found, but I find there are distinctly fewer people discussing more advanced topics, or they are much harder to discover.
2. Audio/Video just isn't as information-dense as a book can be.
3. YouTube and podcasts tend to be much more "infotainment" than "education". And sure, we can find lectures on there, but students get lectures in school too.