Road trauma has been steadily reducing in recent decades with improved safety built into cars - think airbags, stability / traction control, stronger cars etc....
I think that is part of the cause of the indifference toward cyclists.
Most people don't like being under stress and will avoid it if they can. With all those safety advances in motorcars in the last half century, motoring is a much more tranquil, less stressful endeavour than it once was. So when a motorist see a bicyclist on the roadway ahead of them, it would be stress-inducing to acknowledge how exposed and at-risk they are, so they choose either to rationalize that cyclists are every bit as safe as they are, or to ignore their presence altogether.
Furthermore, studies (I don't have the time to look them up and post references just now) have shown that motorists will leave more space when overtaking cyclists who are obviously women, and will leave more space for un-helmeted riders. Which evidences conscious thought on their part, and in some instances supports the theory expressed in Peltzman Effect.
Clearly, motorists have diminished respect for anything they don't fear, and they don't fear anything that doesn't present risk of serious injury to them if they should collide with it. Which is why they habitually overlook cyclists and rarely overlook lorries and motor coaches.
So I would theorize that the safety of the modern motorcar indirectly serves to put cyclists at greater risk because the worst damage we can inflict if/when they collide with us is a dented wing or cracked windscreen.
I don't recall ever having seen one but I would love to see a study of the overtaking habits of motorists who have a close friend or relation who is a bicyclist or motorbiker, because I would conjecture they are more likely to see our lot as beings that will squeal and bleed if injured than someone who thinks of all cyclists as strangers or alien beings.