Driving a car comes with a profound sense of power. Most people see bicyclists as poor people, faggots, hippies, foreigners, etc. Mix power, a degree of anonymity, and a group subhumans to hate, and you get people acting in a way that is incredibly dangerous.
Also most cagers should not be on the road, and barely avoid killing people when dealing with the other people on the road piloting one ton boxes covered in reflective, bright colors. It makes them incredibly stressed having to drive around far smaller, harder to notice objects when they barely avoid killing people under far more forgiving conditions. Stress just makes the anger against the easily scapegoated group all the stronger. After all, *we* are the ones making their lifestyle, the correct lifestyle, more challenging.
Studies show more people unironically think that bikers are subhuman than blacks or jews, so there's a good chunk of motorists who would probably kill you if they thought they could get away with it.