> Not necessarily motivation in the sense of "why is this important", but motivation in the sense of "we are beginning a three page proof. Let me give you a paragraph to give you the outline so that you can fill in the details yourself, rather than having to read all of the details just to reconstruct the outline."

In graduate school this was the most frustrating aspect of paper reading (and writing). It makes sense why it exists however. Papers on mathematics in particular are laser targeted to a particular niche. As the science progresses you need more and more bespoke knowledge of previous work to even start the paper you're reading. There's an implicit assumption you've done your homework, so to speak, and authors likely feel there is no need to provide such a summary. Since, of course, if you don't have the pre-requisite knowledge the paper isn't targeted at you anyway.

Some of it of course is simply a pride thing. There have been many times I've felt the lack of exposition was a way to say "I'm better than you". I have no evidence this is the case but it would not surprise me.