We are trained to shoot to stop the threat. Now the very nature of the threat is life threatening, so it needs to be stopped immediately. The most effective way to stop it quickly is to shut down the cardiovascular or nervous system. The side effect of this unfortunately usually means death. Death isn't the goal, but a likely outcome because of the nature of the trauma inflicted.
The idea of center mass means the largest target area available at the time. Usually means the torso, which also is tightly packed with organs. Damage to the organs means that the body shuts down faster, which is the goal. At the same time center mass could be anything. It was last year that the aggressor was fighting a cop hand to hand with a knife. His partner shot him in the leg because that was the largest clear target he had. That was 'center mass' for that incident.
As for effective shots on a car, as someone else stated, hollow point bullets do not react well to paneling, windshields, etc. Also the human head itself with it's curved skull will more often than not deflect rounds. Simply put handguns are defensive weapons, not offensive.
"It takes a big man to admit when he is wrong....I'm not a big man" Chevy Chase, Fletch Lives.