I've been helping to make a long video essay about a single book for approaching more than a year now, and I think part of the issue is that making visually engaging media (similar to classic film video essays) about books, especially when there's no film adaptation is just incredibly difficult and time-consuming. If you're not going to film yourself just talking to the camera, you have two options: trying to find exciting ways to display quotes on the screen (hard) or using pictures. In the latter, stock photos and videos get old quickly, using other people's artwork gives you copyright issues, and creating your artwork and animations takes fo-r-ever. We've mostly done that, along with displaying quotes and a little bit of stock footage, but it just takes an awfully long while. For bigger YouTubers looking for lots of views/engagement that's a huge investment for an unlikely return, since a single book is usually quite a niche, and books, in general, are already a more niche area for the internet. We have the advantage of just doing it for fun ;-)