I think it boils down to people getting it in their heads that driving is a right and not a privelidge in so much that they will keep on driving if they can't see, can't afford insurance, car fails its MOT or whatever. How often do you see on those late night police programmes with people who repeatedly drive after being banned!
As Adrian said people don't self regulate because they fear the consequences of being stopped driving.
The HGV driver who killed my mate from work apparently broke down under questioning and said he'd suffered from poor eyesight and a sleep disorder for years but had hidden it for fear of losing his job... instead of which he killed someone as a result of refusing to admit he was unfit to drive. It dragged the court case out for months and put my mates girlfriend and family through hell...