It really depends what you mean by making it "beginner friendly".
A lot of work goes into refactoring and cleaning up code, making things cleaner, adding documentation, adding static and dynamic checkers for locks, memory errors, and various other things, making useful common data structures and code into libraries, inspection and debugging tools, reimplementing as much hand-coded assembly as reasonably possible in C, adding a staging area in the tree and moving to a distributed vcs, there are automated CI tests that get kicked off when you post a patch to a mailing list.
In my opinion it is far nicer to develop for, and a lot of code is far nicer to read than it was 20 or even 10 years ago. It's simpler to write some basic hello world filesystem or device driver module because APIs have become a lot cleaner and there are a lot more checks. It is also now far easier let's say to develop a high performance multi-queue block IO subsystem that is mostly lock free and can steer interrupts to specific CPUs based on context and have all the platform specific PCI and interrupt issues and the different memory ordering behaviors of different CPUs all just work. Linux already has one of those though.
Outside of device drivers and new hardware enablement for really simple device types or minor updates to an existing device type, what the kernel is doing now is vastly more complicated. So it is a lot of work to make a significant useful change. This isn't because there is no interest in making it beginner friendly, it's because a lot more of the "easy" things are done.