Lots of brain connections work with one sense controlling how another is processed. In a car you can talk to a passenger all day and drive fine. But answer your phone, and suddenly you're absent and distracted.

I experimented to find out why. When I was talking to a friend in the passenger seat, I held up a card to prevent myself from seeing the passenger out of the corner of my eye - and suddenly I was absent and distracted.

Maybe its because when we don't see someone we're conversing with, we have to create a mental model of them to compensate. Because we're built to see the people we interact with, and our brains may just work that way. And the work of creating the mental model interferes with driving (prevents processing of the alternate mental model of the road situation?)

Anyway, for what its worth.