The primary reason is an absence of Mens rea (guilty mind).
For the vast majority of crimes, you need to have both the actus reus and Mens rea.
With pretty much all driving offences, there is no intent, premeditation, planning etc. Nobody sets off on a journey with the intent to kill, or cause really serious harm, so it is problematic from a legal perspective.
I also suspect that a lot of jurors, to an extent, have a sense of ‘there but for the grace of god’ about driving offences; most drivers have experienced a momentary lapse in concentration that resulted in a near miss.
The cases you describe are extreme, but I believe the reasons given are why this seems to be the only way you can kill a stranger and receive only a few years in prison.
I do agree with pp that when someone has caused death by dangerous driving, imprisoning the driver just causes more damage in the big picture. Is it really a good thing for 2 lives and families to be ruined, when the driver is (usually) of previous good character?
Given the absence of planning/intent for driving offences, the threat of a custodial sentence is of limited deterrent value in my view.