What they're missing is the transitional half-sentence or so, that logically connects the small ideas they're working with. They're expecting the reader to just make the same logic leap they did, and they can't rely on that. Readers are whimsical creatures, and they go in all sorts of mental directions. They need to make the chain of thought explicit and clear.
I think if I explain this, it will help. I hope!
(Am a bit embarrassed that it's taken me this long to figure this out, given that I've been teaching composition, in one form or another, for almost fifteen years. Ah well.)