It may take a bizarre spill for a cyclist on a straight road to fall into the path of a car passing five feet away, but it would be an even rarer one to be in the next lane, so why risk it? It just seems like lazy driving, increasing the risk to the rider because one can't be bothered turning the steering wheel a little more.
I think it's human nature and perception of risk.
If I am driving on a normal 2 lane road and want to overtake the car in front, then obviously I have to pull into the oncoming traffic lane to complete the overtake. As I plan and execute the maneuver I am acutely aware that I need to have sufficient distance to oncoming traffic to safely complete the overtake and that risk is heightened dependent on the amount of road I can see in front of me. I'm at speed, so covering ground quickly. My natural instinct is to want to get back to "my lane" as quickly as possible.
Now if I apply the same logic to overtaking a cyclist, whilst I could do a whole lane pass (and there are plenty of reasons why I should do this - cyclist is at a slower speed than a car would be etc) the tendency is not to want to do that but to minimise my exposure to the oncoming traffic lane. Thus the more common overtake is the half lane overtake. It minimises exposure to the other lane, reduces the distance that I have to move off line etc. People tend to go with what they feel is right, rather than what might be better.
In my experience, the "whole lane" overtakers tend to be more nervous (e.g. P Plates), or more elderly drivers.