… I do think that “driver’s licenses” is more often due to simple carelessness rather than a considered decision to consider “driver’s license” as a fixed phrase or a single lexical unit.
I would think that the most common explanation would be somewhere in a middle ground. No great lexical analysys is required for most of our speech or writing. We just use what comes from our internal grammar and what looks good to the eye. While I agree that a case could be made for either version we don’t generally make cases when using language unless someone comes along and challenges what we have been using all our life. Once I got in to a long involved discussion about that validity of the phrase “different than” and the more we
argued discussed it the more it seemed that none of the stardard options “different (from, than, to)” really made any sense and that one could only really say that one thing differed from another (in some sense). I now try to use “different nor” as an option as valid as any of the others.