I believe a lot of the problem is that the major vendors believe they'll expose themselves to patent lawsuits by making it easier for aggressors to make a case.
Besides any patents there are issues were Nvidia/AMD do not write every piece of their drivers. They can't unilaterally open source pieces of code that they don't even own the copyright on.
Patent thickets and minefields are common in the hardware world. Perhaps you remember the lawsuits and "cross-licensing agreements" between Intel and AMD years past?
Plus, it doesn't have to mean that it's hard to do in the first place. Being able to grep someone's source code tends to make it a bit easier to find that obvious usage of the 'if' statement (or whatever) that you have a patent on.
