Context is often missing. Or basically, the four quadrant. People often talk about the known knowns, known unknowns and unknown unknowns, but most people forget the ( possibly most important one ), unknown knowns. And because the expert practitioner didn't even known they know ( something that they think is obvious but didn't know others have no idea ), it is hard for them to teach.

Unlike in Universities, where each student are given the same resources for them to read through and progress, most of the teaching in real world ( non-academic settings ) does not ofter the same path.

And without those foundational knowledge, a lot of discussions would be broken because the assumption of you knowing is untrue. For example, half of the software developers have absolutely zero idea how computer works. But they somehow think they do, as witness in the recent HN discussions. This makes teaching and discussions on a subject enormously difficult.

Edit: OH this is great. I just discovered there is a term for it in the comment below. The curse of knowledge. https://en.m.wikipedia.org/wiki/Curse_of_knowledge