top down designing

I have been reading "Understanding by Design" by Wiggins and McTighe(1998). They speak of using "essential" questions to plan your unit. These are 2-5 questions set up ahead of time that students should be able to reflect on daily and build a response to throughout the unit. They suggest posting the overarching questions around the room so that it reminds students of the "big ideas" for the unit/course/discipline. They also suggest building assessment beforehand that address the exploration of these questions. Again, I am reminded of the importance of the design down approach to unit planning. Hello! Of course! So why does it not happen more often? A mathematical example they give in the text is "Why is it true a triangle always has 180 degrees? How can we say that for sure?"