Maybe it's built in "pyramid" like this because:
1. Knowledge is hierarchical
2. Calculus has a longer list of skills that needs to be mastered before learning the subject.
3. Historically, calculus came around first (am I right?). This might not be a coincidence.
Understanding (the path to knowledge) might be hierarchical but knowledge in general isn't. Organizing knowledge in hierarchies is a conventional teaching technique, and that has some self-fulfilling qualities: if you teach people in hierarchies, they'll see the world that way.
