Two points.
1. Most of the time criminal charges are not filed, because the incident is not believed (by the DA) to be provable in a criminal case (or because they don't think a jury would convict) since the standard of proof beyond a reasonable doubt is fairly high and the costs for such prosecution are high as well.
2. Even when criminal charges are not filed, the victim has the option to pursue civil liability which has a much lower standard of proof. In a country that awards millions of dollars to idiots who spill hot coffee on themselves, it wouldn't be hard to punish the driver by bankrupting them. Personally I think this is the appropriate method to hold a driver responsible in most of these incidents.