Often, if the concept is presented in a more complex way the reason is that the author wants to emphasize and explain how the concept relates in a non-trivial way to some other deep concept; thus you learn a lot more than when the author explains things in the most simple (and shallow) way.

IMO the most common reason why something is presented in a more complex way is that it is badly explained.

Of course, most common or not, each case is different.

Also speaks to a lack of understanding on the author's part; people who truly understand some subject are generally much more adept at explaining it in simpler terms – ie without adding complexity beyond the subject's essential complexity

I don’t see how that is beneficial. If a simple concept relates to a complex one then explain the complexity, don’t add it.

> If a simple concept relates to a complex one then explain the complexity, don’t add it.

So write a text of at least 500 pages to explain the complexity. :-)