Primary issue is IP rights. For example, DXTC (S3TC) is patent-encumbered, but many GPU features are not only patent-encumbered but are trade secrets - vendors like NVIDIA have in the past had trade secret, vendor-only extensions for things like a special texture filtering algorithm for shadow maps. So in many cases a driver vendor may not even be able to open their source if they want to, because it contains proprietary trade secrets from a third party, where they're using third party code to compile shaders or whatever.