While I'm not sure I agree that most people die due to fire after an accident, I do agree that fires do kill some motorists. The idea of an automated fire system is intriguing but not very practical. Buildings have this type of system installed in the form of sprinklers or halon extinguishers. A vehicle does not have adequate space to store enough water to extinguish a gasoline fueled fire. While other types of extinguishers like halon are lighter and smaller, they also have the unfortunate side effect of suffocating humans. A person in a building fire is likely to be able to evacuate the building. A person in an automobile accident is far more likely to be unconscious or otherwise unable to escape the vehicle. A fire extinguishing system could be potentially harmful to the injured occupants.
I think many car companies approach this type of issue from another direction. They attempt to make vehicles safer by reducing the risk of fire in the first place. Gas tanks are as far from the engine as possible for this very reason. Most cars have a crumple zone around the gas tank; in the event of an auto accident, the gas tank will be protected from punctures by this crumple zone. Car companies also try to ensure a person will be able to safely evacuate the vehicle and that electronic components will not spark a fire if the vehicle is in an accident. Newer model cars are far less likely to combust than older models because car manufacturers are always looking for ways to make vehicles safer.