Presumably, the main reason for this is that governments really care about the lowering of drivers' standards. Ensuring that all drivers are appropriately educated (i.e. is seen as very important. By contrast, the governments don't care that much about the lowering of academic standards. If they would, they would long ago have replaced a present grading/certification system with one where students are tested by independent bodies, rather than by the universities themselves.

I strongly agree with the main message of your post but this is just lazy thinking. The obvious answer is that setting a universal standard for college courses is much harder than having universal driving licence standards. Everyone agrees that you should not drive through a red light. But what if one student comes from the Austrian school of economics and another from the Chicago School? How are you supposed to compare the students' knowledge objectively? If the government gets involved in this process people will inevitably cry that the government is trying to enforce their politics on the students. You can get around these issues but it's still a messy process.