It's been shown often that people are very capable of thinking of others as not human. Now wrap them in a nice metal bubble (car), and put the other human into

It's been shown often that people are very capable of thinking of others as not human. Now wrap them in a nice metal bubble (car), and put the other human into a weird pose on a metal frame (bike) and the division is even easier. I'd then also imagine it's a coping mechanism for the guilt/stress drivers face worrying about cyclists, given the difference of damage in a crash, so they think of the cyclists as "a cyclist who decided to ride up next to me" rather than "a human on a bike trying to get to work".