It would be completely possible to pass the drivers test and have a license for the rest of your life without ever getting behind the wheel. Many people have licenses and seldom drive.
I think if ENOUGH people who seldom drive got injured in accidents, there would be a push to tighten up the driving license requirements. For example, annual re-testing might be imposed. I believe that in some states, elderly drivers are required to undergo more frequent re-testing. It seems to me that, whether it's supported by statistics or just an unfounded fear, dive operators implicitly believe that enough people who seldom dive are getting injured to warrant checking their skills out or taking other precautions so as to avoid taking them on dives that challenge their abilities.
Anyway, the driving license analogy has to go back to the question of who is liable. If the person who seldom drives goes off driving by himself and nobody else is involved, it's clear the liability for an accident would fall on him as opposed to some other party. But if we consider some hypothetical scenario in which some other party, such as a car rental agency, could be argued to have control over what roads the driver takes, how fast he drives, etc, then it's less clear whether that other party could get off the hook for liability in the event the driver has an accident simply by saying "he showed me a valid driver's license." If that kind of scenario were common enough, then I think we would see car rental agencies doing more than just checking the renter's license.