@Mame,
I've had people talking on phones, change lanes without looking/comprehending that someone was in their blind spot.
I think it's mostly about the pure distraction, one is concentrating more on the topic they are discussing rather than the other cars on the road.
Personally I don't like having chatty passengers in the car when driving. They create their own distractions, like "oh look at that cool blimp!"
Also, although they may see the car in front of me, the passenger may not realize I'm not stopping at a yellow light because the car behind me is traveling too close. If I'm on the phone, I would probably react more to the color of the stop light (auto conditioning) rather than being focused on the driver behind me.