Because they are none too bright and it usually other people who suffer for their foolish actions.
Inconsiderate dolts that think that their phone chat is more important than other peoples safety.
I would ask them but they are busy talking.<br />
The same reason women put on make up while driving.
Same reason why people smoke knowing that it's not healthy. People are risk takers.
I do it when its important, but im not going to pull over to take a phone call