I bow to your greater knowledge of marine certifications; I can't address that. However, I don't view the standard driving test to be an indication of competency. Things have become more stringent these days; I remember that when I got my license (1976), you could show up at the DMV, take the paper test and a very brief road test and walk out of there with your license. My kids had to not only take the road and paper test, they also had to have a certain amount of driving hours documented as well. But even with that, I don't consider newly minted drivers to be "competent".Actually, most marine certificates are not about education, but are certificates of competency, unlike an educational degree.
How can any agency certify competency if the holder never has to do anything more than pass the test? You have a written test and a driving test to drive any vehicle, and you aren't getting a big rig license driving a sports car.
I've heard that the RYA certificate is actually more along the lines of what you are thinking: a written test, but also accompanied by a real competency evaluation. I recall reading a magazine article where the RYA candidates had to spend multiple days aboard a yacht with the evaluator who graded them on their skills.