@Aguablanco1, I'm glad you admit the absurdity of those analogies.
In renting a car, the rental agency has no more involvement once the customer drives off. The driver can drive wherever he wants. On a dive boat, the crew, possibly including a divemaster, are involved with the diver from the time the boat leaves until it returns. The dive boat can choose to take divers to easy sites, challenging sites, etc. Also, the legal system has pretty well established that a rental agency is insulated from responsibility for accidents caused by the driver/renter who presented a valid driver's license. In contrast, in the scuba world a c-card may not be given the same weight in a lawsuit. A c-card is evidence that the dive op acted reasonably in serving a diver, but it may or may not be enough by itself. In other words, as much as you say you'd like it to be this way, it isn't clear whether a valid c-card is by itself sufficient to insulate a dive op from liability. I think this uncertainty motivates dive ops to look further.
Also, I would guess that the majority of people who rent cars drive regularly. In contrast, a great many divers show up at dive ops having done little to no diving in the past year. Some do just one dive vacation annually. There is a lot to remember and a lot of muscle memory involved in diving. Driving a car is a pretty common activity, and even people who don't drive every day are exposed to cars, driving, and the rules of the road by being passengers, observing, etc. A driver who hasn't driven in a year doesn't forget which pedal is which, because he's exposed to cars and driving all the time, but divers who haven't dived in a while fumble with which button is the inflator and which is the deflator.
As for renting a boat, I have to admit it has long amazed me how little is required to rent a small power boat. Depending on where the boat is going to be taken, I think a boater really should have to take a short test to show they know the rules of the road. I guess it all depends on the circumstances. Lake versus ocean, etc. Renting a sailboat as a bareboat charter in the Caribbean, the rental outfit may very well want to see your logbook.
edit: killerflyingbugs posted while I was typing and said pretty much the same thing in many fewer words!