I was listening to an older episode of "Crash FM" (The Burnout/Criterion Games official podcast) just the other day, and someone wrote in to them asking why not.

They actually gave a very good answer that made a lot of sense. They are trying to make Burnout appeal to many different people, different age groups, and above all, make a fun game. By putting a driver in a car that is flying through the air, they are adding something violent to the game, that some people might not want to see. They said that so many people die in car wrecks, that adding a person flying through the windshield of your car could offend people unnecessarily. So, I guess I kind of agree with them.

They also said that they spent so much time on detailing every little thing, that if you add a driver, they have to add details to him like turning the wheel, throwing his hands up, facial detail, etc., they just did not want to bother themselves with all of that, when after all, it's just a driving game. Not a crash-test dummies kind of thing