I hate to think what would have happened if the car had had electronic steering. But I would hope that the designers take into consideration the possibility of a sudden and complete power loss while driving. The answer is that combat aircraft receive a great deal of care and preventive maintenance. Cars, on the other hand, are often driven without being properly maintained and their systems are not repaired until they break down. Mechanical systems are fairly reliable and degrade faily smoothly.
Electronic systems tend to simply stop working.
In tomorrow's car, you'll drive into the oncoming lane without any control at all. A wild transfer in a program or a dropped bit may result in strange behavior. Wet brakes may fail in either case. For that matter, in each case there is the possibility of overreaction. A friend of mine once told me that the first time a prototype F16 was taken out on the runway, its test pilot tried to retract the F16's landing gear while on the ground.