It's some combination of loss aversion and the associated guilt that comes from negative consequences of positive actions.

There is a trade off: fully automated transportation (planes or cars) will eliminate routine operator error, like falling asleep at the wheel. On the other hand, there will also be corner cases which humans could (sometimes) handle that the autopilot didn't.

And human psychology means that not only will we weight the losses more heavily than the gains, but when losses happen and people die, we will feel unimaginably guilty over the losses, no matter how many people were saved.