It's a problem of incentives. Books sell much better than post-it notes, so if you have a brilliant insight you better come up with at least 100 pages of filler to turn it into a book. Or at least make a one hour conference talk, a 10-minute youtube video, a 7 page research paper, or a three page blog article.

The only medium that rewards getting to the point is casual conversation, which is one of the reasons why on conferences you often learn more in five minutes of talking to the right person than in the entire rest of the day.