there are different reasons for different car manufacturers involved.
here are some basic points to licensing cars. not just about damage models.
1. how much money is the softco prepared to pay??
this may seem trivial, but why should the developer have to pay? if you look to films, every car manufacturer want their cars shown off in the latest hollywood blockbuster. the car manufacturer benefits from the exposure just as much as the film benefits from a cool looking car. sometimes you'll see the car manufacturer pulling out all the stops to get a prototype/ concept in a sci-fi movie etc.
2. how does it contribute to the image of the cars/ brand?
do i really want my car to be associated with a game that's about murder, rape, drugs, guns etc?? do i want it to fall to bits the moment it smashes into a competitor's car? would i be happy if it doesn't accelerate the way it should, brake the way it should... do i really want it to swerve out of control like that???? safety is very important.
3. sorry, we don't deal with video games.
you'd be amazed how many car companies don't really have a person/ department that deals with something as foreign to them as videogames. "uhm...hang on a second, it was in the news that such-and-such a game influenced a child to kill himself the other month, i don't want anything to do with games at the moment".
...it's tough, but it's about changing people's perceptions of videogames and forming better working relationships with car manufacturers.