Very short - AMD made an abstraction layer, to share effort between Linux and other platforms (i.e. Windows and etc.). Kernel/DRM maintainers don't like that, since it causes several issues detailed in that thread (harder to understand logic of the driver, slowdown of DRM improvement itself, indirect workflow of AMD developers and so on). For the reference, DRM here is Direct Rendering Manager[1], nothing to do with crooked Digital Restrictions Management.

1. https://en.wikipedia.org/wiki/Direct_Rendering_Manager

Nvidia uses largely the same driver code for both linux and windows in their proprietary driver (I believe they call it a unified driver).

AMD tried the same in their open source driver and were rejected by the kernel maintainer. Unified drivers have code sharing advantages but don't follow the practices of the linux kernel.

Short version: AMD as a company is dysfunctional. Perfectly happy to throw away $300Mil on failed acquisition, unwilling to hire sufficient number of competent driver developers.

Result is no people to do the required work.