For once, OpenGL/DirectX drivers contain a shader compiler since the compiled form of shader in these APIs is in IL bytecode and it has to be compiled into the actual ISA code to be able to run on the processors inside the GPU. Such a compiler is pretty complex piece of software and even if it did not have licensed code opening it would expose you to all kinds of patent trolls.
Also, the DirectX drivers (and likely OpenGl as well) contain special code paths for many games/apps to improve performance. When they are telling you to download the latest drivers for your new PC game it's not because the new drivers fixed bugs and the game won't run (though it's also the case sometimes), it's because the new drivers have an optimized code path for that game. Developing these code paths costs money and giving them away for free may save money for the competition since the hardware is pretty similar and most of these optimizations are selecting the best hardware representation for an API entity.
Also, the DirectX drivers (and likely OpenGl as well) contain special code paths for many games/apps to improve performance. When they are telling you to download the latest drivers for your new PC game it's not because the new drivers fixed bugs and the game won't run (though it's also the case sometimes), it's because the new drivers have an optimized code path for that game. Developing these code paths costs money and giving them away for free may save money for the competition since the hardware is pretty similar and most of these optimizations are selecting the best hardware representation for an API entity.