I have no data to back this up but just taking a stab in the dark - a possible reason might be because people generally tend to prefer learning from someone talking about the subject matter?

I know most all of us here are techies and very used to cracking open books and documentation and text tutorials to teach ourselves stuff, but many people are not like that and especially if you're new to a subject, sometimes books just don't help things to click as well for some reason.

There's probably something to do with the way material is structured and presented differently between talking about it and writing about it, but I wouldn't know what to say about it.

I dunno, just a guess because it's an interesting observation to think about.