It could mostly be that they just don't see you. They are used to looking for cars and they may be staring in your direction without noticing you. As a car driver I've sometimes almost failed to see bikes, even though I ride a lot myself. You have to ride with the awareness that they may be looking right through you.

On my bike, I pre-emptively ring my bell a lot when I see I am converging with a car, to try to raise their alertness.