There is far more to the actual software of pbrt then what is described in the book and the book is huge. All the mechanics of the architecture are really something you can only find out by looking at the actual C++ files.
>A method known as literate programming combines human-readable documentation and source code into a single reference that is specifically designed to aid comprehension.
Where are these C++ files which are not included in the text?
And now imagine maintaining the code of PBR, and with more than 5 people working on it at the same time. Literate programming is great for reading, although I prefer having "real code" (which can normally be extracted, that's not a problem) to read too.
I'm not saying it is for everyone, or for every project. My point was that "Literate programming only works for small scripts and narrative documentation" is far too wide-sweeping of a statement. There are plenty of non-trivial examples of its use, and it obviously "works" for some people.
It certainly does, but these "some" really are exceptions. Just try it for yourself, edit PBR and compare the experience to "just" editing the source code of PBR (yes, of course, not being used to something doesn't help). To be honest, I haven't looked at PBR for more than a decade so maybe I would succeed nowadays, but I doubt that.
I guess realising that literate programming is literally [sorry] writing a programming book should be enough of an argument to know that it isn't suitable for most people. And no, a white-paper is not a "programming book" suitable for most people to read ;)