If you put 4 people in a NASCAR, it would be just as deadly as any other car (even assuming it were designed with four seats). Training and context are the main factors in the relative safety of NASCAR.
Race cars are engineered to protect professional drivers from high speed collisions in a race environment. Race car drivers don’t weave through mixed speed traffic on the freeway and tailgate people who stop suddenly. NASCAR drivers don’t cut off big trucks or drive at excessive speed through blind turns. If they were used by everyday folks for daily errands and commuting, the death and injury rate would only be modestly lower. NASCAR drivers do not drive recklessly on the track, which is a contributing factor for a distressingly high percentage of accidents.
Similarly, a regular cars driven in a race environment would only be moderately more dangerous. All the cars are traveling at roughly the same speed and heading in the same direction. There is no two-way traffic, preventing head on collisions, and no intersections for people to run red lights through, preventing t-bone collisions with crossing traffic. The race track is a controlled environment, with fewer opportunities for failure. There are still spectacular accidents, but roll cages, five-point harnesses and helmets, and protected track barriers can mitigate most accidents.
Stepping a little further back philosophically, no normal person is going to wear a helmet or five point harness willingly. It takes a considerable amount of time to get strapped in and out of a five-point harness properly. Too loose, and it is useless. We have to ticket people to get them to spend 5 seconds buckling up! Spending several minutes carefully adjusting a complex harness is not going to happen. Even it if did, the harness is designed for collisions at 150 mph, not normal highway speeds. It is overkill, and using them would make people less safe, because they wouldn’t bother.
In essence, it is all matter of personal responsibility. People must drive safely and competently. If people drove race cars unsafely and incompetently, the race car would be just as unsafe or even worse than a regular car.