If these tests worked, there wouldn't be so damn many terrible drivers on the road. But there ARE tons and tons of shitty drivers on the road, so we know they aren't effective.

If the DMV began diligently evaluating everyone's ability to drive, there would be a lot less people getting cleared to drive, which means a lot less people paying the fees to get licenses plates and tags. I'll bet insurance companies wouldn't like it either, since you wouldn't have much luck selling insurance to people who can't legally drive.