> only to discover that you’d absorbed what amounts to a few sentences?
This is akin to saying that "F = MA so why do I need a whole series of lectures and chapters in a textbook to explain it!" Understanding the basic idea in physics that F=MA involves building a variety of intuitions about what this really means, what it entails in the real world, and how this influences the ways we think about this powerful idea. You need to see examples, experiments and applications of this idea for it to make sense, even though it can be perfectly reduced to 4 characters. Once the student of physics has all of the information in their head a vast amount of complexity can be reduced to F=MA.
An even more powerful demonstration of this is of course Einstein's famous E = MC^2. A pretty simple assertion, but really unraveling what that means and entails is easily many years of study.
On the other end of the spectrum, there are many books that genuinely defy reduction in this way. Hegel and Derrida are notorious examples of these. Scholars of either would tell you that not being reducible is an essential property of what each of these writers is trying to say. However I suspect the author of this post would very quickly become frustrated with these works and beg them to "get to the point!".
And another perspective outside of these are the "Desert Fathers", the early Christian ascetics, who would spend years meditating on single passages from the bible.