Why exactly is it so difficult to polish it up, document it properly, and translate as necessary, that it can't be accomplished by a few smart engineers in a few months?
https://salsa.debian.org/Mobian-team/devices/kernels/sunxi64...
The diff is 13.4 MiB and is over 200,000 lines long. It includes it's own AES and SHA-256 engine. Trying to polish this up would be close to years, and a good kernel dev is not cheap.
The datasheet only gets you so far, especially for undocumented hardware quirks, and the vendor is in a much better place to understand what's happening and fix them. It's an active process of reporting bugs, investigating them, and getting fixes merged upstream into the vendor's BSP, at the very least. This requires active maintenance, which means engineers on the vendor side. That's where the support contracts kick in.
Upstreaming to Linux on top of all that means that you actually need to support and validate the set of features your chip claims to support. That means more thorough test and validation, code that meets the standards of the kernel, and more engineers to actively maintain the code. Not to mention the language barrier for Chinese chip vendors.
Of course, any upstreaming efforts are great and I think things are trending in a good direction. Just pointing out that there are reasons things are the way they are. Fabbing a chip that isn't completely buggy and people want to buy is already a small miracle- better software support can always wait until the chip becomes a hit.