Why not the leg, they are teaching shoot to kill now.
Police are not trained to shoot to kill. We are trained to shoot to stop.
There are a lot of factors that make center mass the optimum shot placement to achieve this.
1. It is the easiest spot on a human body to shoot. It moves the least in relation to an arm, a leg, or the head.
2. A missed shot can easily hit an innocent bystander. A 9mm projectile can be lethal for up to 1km passed its intended target.
3. There are very few places on a human you can shoot someone that is guaranteed to not be fatal -- unlike the movies, your shoulders, legs, and arms all contain arteries that if severed could be fatal in a few minutes.
4. 80% of all gunshot victims survive so the odds of a fatal shot are against the officer.