You're asking a question with a very complex answer.
I'll start with a short answer - AMD wants to have at minimum a reasonably good baseline functionality on linux, out of the box. If that answer doesn't make much sense, please read on for the complex bits.
To provide some historical context, Linus gave a VERY public, VERY brutal rant on NVIDIA and their drivers four years ago.[0] At the time, NVIDIA's closed drivers were an opaque blob which basically reimplemented the entirety of OpenGL. These collided with everything else.
The open drivers (nouveau) were slow and far behind in features. The situation was bad enough that you couldn't necessarily even install a linux distro on a system with NVIDIA chip because the open drivers wouldn't work well enough for X and/or desktop environment to initialise properly, let alone remain up and functional.
In effect, if you wanted to run linux, you were best off without NVIDIA.
Fast forward year and a half. NVIDIA had come out and committed to improving the linux driver situation. They still couldn't open source their current-generation drivers, but they had realised the horrible reputation of their hardware on linux was going to be a persistent PR nightmare, and thus an existential threat to their growing mobile division. A space where they were up against Imagination Technologies. (Don't get me started on ImgTec, please...)
NVIDIA needed to get their hardware and software processes aligned in a way that they would work reliably out of the box on linux, everywhere. Even if the user only needed to keep the freshly installed or upgraded system up long enough for them to download and install the closed drivers, the system really should not break. Incidentally this meant that even the open drivers should be "good enough".
Over the past 3+ years, the situation has improved. NVIDIA has managed to shed their reputation of being completely broken on linux, they have worked closely with kernel folks to get their more recent hardware supported sensibly out of the box and in the process (I believe) they have managed to reduce the code delta between their windows drivers and linux drivers.
It helps that during the same 4 years we have had OpenGL on mobile drive the separation of duties between EGL and GLES (v2+). These changes, with all the refactorings, have also provided a cleaner split on desktop - to the point that it is no longer absolutely necessary to provide full OpenGL implementation. You can, for most parts, expect that EGL just works; that DRM and KMS both just work; and that your highly optimised GLES implementation can happily live on top of these layers.
As a result, your closed driver offering has less to override. Of course it's going to be less unstable!
Disclosure: in my previous job, I helped integrate couple of EGL+GLES driver stacks with Wayland on mobile systems. I learned to hate mobile GPU drivers with a passion.
P.S.: I haven't read enough about Vulkan to know if it improves things or not.
0: https://www.youtube.com/watch?v=IVpOyKCNZYw