The reason for the whole phobia is due to our under training as youth. As we first encounter the bicycle as young children, we have poor motor skills and poor balance... our instructors (parents) see this and suggest that we should ride on sidewalks and "watch out for cars." This instills in us a sense that we should be paranoid of cars when we are out playing with our "toys."
Most adults do not progress beyond that "watch out for cars" training and the basic training on their bicycle "toys."
Wiser youth later understand the need for exercise and see the efficiency of the bicycle as a grand and wonderful thing, those same wiser youth also learn through trial and error that all traffic has a place on the road and that co-operation is key to any and all traffic situations.
There are also those few youth who are less wise who later feel that the motor car is their new "toy," and that they can play "road racer" on the streets. Often these drivers are eliminated in a Darwinian manner as they push their motor cars beyond the operating envelope in dangerous maneuvers such as speeding, drinking and driving, road racing etc. Some of those less wiser persons survive into adult hood and continue to drive in a reckless manner, not in co-operation with other traffic. Those motorists should be avoided.
The majority of motorists however never progress beyond the toy view of the bicycle, and thus retain their fear of the motor car, and cannot envision bicycles as traffic... Nor are they trained in any other manner along the way. While they may be taught traffic basics in some six week class called drivers' ed., they hardly have time to build proper motor skills, much less gain all the finer skills of traffic handling... much less be indoctrinated in the facts regarding their once beloved bicycles, as traffic. Thus the average adult driver, once taught to keep out of the way of cars while on a bicycle, retains that knowledge as a phobia.