I spent a very brief time as a contributor to WP internals (not core by any means but a handful of contributions) and not only do you need a lot of internal knowledge to do anything outside of point-and-click, like you said, a lot of the core folks had never worked on any other large software projects, didn't do anything outside of WordPress, and had very little desire to make sure what they were doing was sound from a software engineering / code quality standpoint. It's a pretty great example of the big ball of mud / spaghetti code cliche.