I'd guess that a big reason people don't stop is that even if it were an accident they'd be sued to hell and back. Even if no one were severly injured they would be raked over the coals and suffer for years. I truly don't believe that people should flee after a hit and run....but this is just my take on "why" people may not stop.
Ding Ding Ding - We have a winner! Have an accident and just wait for the lawyers to start circling to pick your bones clean, with or without insurance.
And the guess is wrong.
There is a reason that hit and runs are up in California. Read some of the other replies for the answer.
There is a direct correlation between California's rapidly changing demographics and the rise in hit-and-run accidents. FYI: The incidence of leg bail, after injury accidents, is also up dramatically. First responders frequently arrive to find that the injured have fled with the help of their companions.
The principle contributor is culture in California, not fear of civil law or the cost of medical treatment.