More fundamentally, justice is meant to punish the intent of an action, not the effect of the action. From my ill remembered moral philosophy tutorials, I recall the example of two individuals each shooting a gun across a room full of people. By complete chance, one hits and kills someone, the other one misses everyone. The argument is that both individuals should be punished equally, since it is the action that is significant and in their control, the outcome was random chance.
With "careless" or "dangerous" driving, the offence committed by the driver, be it inattention or a stupid manoeuvre, is generally carried out thousands of times a day by other drivers. By random chance, one driver in thousands causes injury or death by the action. In principle, the law should punish equally all the drivers, not just the one who, by blind chance, has caused the injury. Since this is never going to happen, judges just tend to go lightly on the one in a thousand, probably knowing themselves that they are guilty of occasional inattention or stupidity while driving.