They have contractual obligations with other vendors to keep some aspects of their hardware secret. This is due to the DRM requirements they have to face. This is simply part of the reality of being a graphics hardware vendor in this day in age.
Patent issues are another one.
Copyrights can be a issue, but it depends on the vendors and how much they contract out to other businesses.
Trade secrets are another issue. Between ATI vs Nvidia they make or break sales by the quality of their windows drivers. That is simply a fact and is another aspect of business that cannot be escaped.
Those issues are going to be big ones.
Writing new open source drivers from scratch, while combined with not documenting certain aspects of the video cards, can avoid most of those issues except patents.