They can be if they're serious enough. Planes and trains carry enough people/other stuff to usually warrant the independent investigator to step in. Buses are sort of there. Cars aren't.

You can either have a good team of people drill down into the root causes of a particular mass-transit failure, or you can have them suffer the death of a thousand cuts by investigating every nose-to-tail collision on a highway because people get distracted. One's worth far more to society.