Because a difference in scale can become a difference in category. A handful of buggy crashes can be reduced to operator error, but as the car becomes widely adopted and analysis matures, it becomes clear that the fundamental design of the machine and its available use cases has fundamental flaws that cause a higher rate of operator error than desired. Therefore, cars are redesigned to be safer, laws and regulations are put in place, license systems are issued, and traffic calming and road design is considered.

Hope that helps you understand.