The Specifications Don’t Exist
While not commonly used as an excuse, this is the most probable reason for hardware manufacturers not releasing specifications for their devices. The driver team within the company may well be operating from incomplete specs, or even from something a bit closer to the implementation.
If there is no specification of release quality, the only way to release one is to write it. This requirement can be costly and, since it doesn’t directly lead to greater profits, it might be difficult to persuade a company’s management that writing specs is in their best interest.
The existence of good hardware documentation for the driver team, internal or external, usually results in a better quality driver. If lack of documentation is the reason why a company cannot release a detailed specification, some caution is probably advisable when considering relying on that manufacturer’s driver.