Embedded [nVidia] developers "optimizing" games by rewriting and hand compiling shaders to replace shaders in shipped games isn't enough of a clue as to why these drivers aren't OSS for you?

The article is rich with the reasons behind this. Some of it is IP encumbered, some of it is benchmark hubris. All of it is the shroud of secrecy that the graphics market prided itself on coming back to bite them in the ass as their driver stacks are now massive piles of virtually unmaintainable code - code that can't even be rewritten because the companies themselves (well, at least AMD and Intel) are hiding details about how the hardware works from itself!