And they ALREADY have expertise.
A computing expert already has decades of highly detailed experience and familiarity with a bunch of paradigms, uses, and conventions.
Experts are the LAST people that want to read manuals for basic things they already have extensive experience with, like desktop environments. Again, they're busy. Being experts.
So, reading the manual on new tech that needs to be implemented in a complex system—great. Reading the manual on a desktop environment? Seriously? That's the last thing an expert wants to be bothered with. "I've used ten different desktop environments over thirty years. Can't you pick one set of conventions I'm already familiar with and use it, so that I can apply my expertise to the actual problems I'm trying to solve? Why reinvent the wheel in such a simple, basic system?"
DEs should leverage existing knowledge and use habits to enable experts to get their real work done quickly. For an expert, using the desktop is NOT the problem at hand requiring a solution. It's not what they're being paid for and not what they care about. Experts love to learn new things—in their area of expertise.
So sure, desktop environment developers probably love to poke around in KDE's front end, code, and docs. But anyone else? People that are not DE specialists are not so excited about the new learning project that is "my desktop," I assure you. The desktop is the last thing they want to be consciously focusing on over the course of the day.