The problem with Wordpress is that once you need to do something outside of the norm then all of the sudden you need deep knowledge of Wordpress's inner workings.
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.
I recall there being several other also-rans, but am having trouble recalling names.
Once the plugin ecosystem sprung up around it, WP was off to the races.