David49
You have to consider physically disabled drivers, there are many that cannot control their car as a fully able person would.
I knew one man, he struggled on sticks but his reaction ability was much slower than normal, one day he entered a roundabout in town, but for some reason carried straight on to the pavement injuring several pedestrians, one very seriously.
He must have been under close medical supervision, there should be a point where doctors call for a driving review.
Possibly his doctor was not aware that he was still driving? If he wanted to continue driving he may have avoided mentioning that he still drove.
I agree that some people are driving who really should not be on the road, but I doubt that GPs would agree to getting involved in any assessments of driving capability. It would be very time consuming, and they'd probably want to charge for the letter.