1) We tend to pay more attention to what is more difficult[0], and paying more attention is often how we become better at something.
Successful teachers and instructors don’t “chew” everything out and explain in simplest possible terms but rather facilitate an involved process throughout which the student has to make own effort in order to arrive at an understanding and in the end experience an “aha!” rush that is stronger, more memorable and inspiring.
If I am reading a piece of writing and a word takes a moment for me to understand or I’m redirected to a footnote for no obvious reason, that itself might just make me remember the idea better; if I suddenly notice a second layer of meaning, the feeling of connection with the author facilitates deeper understanding. (And if I stopped reading at that point due to insufficient motivation, perhaps that’s how the author intentionally made me self-select out.)
2) Not every idea can be directly verbalized.
One could argue that whatever can be conveyed in writing by words in their direct literal meanings is only sufficient for communicating strictly technical information. Imparting a non-trivial concept, a feeling or emotion requires lateral, indirect approaches—and that’s where author’s choice of words, idioms, rhythm, connotations, anything can play a role. (Think of background music in a film and how it influences your impression; try watching a piece of dialogue while playing an uplifting/energizing piece of music and notice how the mood changes.)
[0] In non-arbitrary ways; it’s a fine line between keeping readers on their toes and trying to be obscure for the sake of it.