With some exceptions like Osmium wrote below, I do not think the authors are using unneeded complexity purposefully. I think complexity does end up happening still because (A) of the reason I mentioned, (B) them being not as good at explaining [1], and (C) possibly added effort required. Of these, reason A does not apply when talking to the authors in person.
I would like to hear if you have further explanations for what is behind this. For example, I read in [2] that reading is much more complex operation for the human mind as compared to seeing or hearing because, assuming you believe in biological evolution, reading and writing came to humans only about 6000 years ago while verbal language and hearing came a few millions years back. I do sometimes find listening to Coursera videos more helpful than just reading the slides, but then feel that this is generally because the slides are often incomplete (they often do not really "say" it).
[1] Many people, especially engineers, often lack good presentation skills. I am extrapolating this to writing since that sounds natural even though I do not have not directly experienced this.
[2] http://www.amazon.com/Designing-Mind-Simple-Understanding-In...