It is a limit on what you can do when driving. Guns are safe when used responsibly and without criminal or evil intent. Cars are safe when operated in a rational manner, not under the influence of drugs or alcohol and without evil intent. Guns have positive uses - hunting, target shooting, self defense - people collect them for historical reasons, for appreciation of the technology and even the artistic nature of some firearms. Cars clearly have positive uses - transportation, recreation, artistic appreciation, etc.
I am not equating the two - cars are more integral and necessary to modern society. But both can be dangerous under certain circumstances so limits have to be placed.