The reason those are investigated is because the airline business has taken all reasonable precautions to prevent them. In that case it is fine to use the word

The reason those are investigated is because the airline business has taken all reasonable precautions to prevent them. In that case it is fine to use the word "accident" to describe an event that occurred despite the monumental efforts of the industry. In the case of cars it doesn't make sense because nobody is putting any kind of organized effort into reducing the mechanized carnage. If car wrecks ever become so rare that we thoroughly investigate each and every one of them, then we can start assuming they are all accidents.