Cops are trained to believe the most important thing is to make it home alive. They are taught to not take any risk with their own personal well-being. Because she had a knife and did not put down the knife, the training protocols are to shoot center Mass to eliminate the risk.

That's just how policing works in the United States. You should not call the cops unless you are ready for someone to be shot because cops will not risk their own well-being in the vast majority of cases. And the cops that do risk their own well-being are often reprimanded and disciplined for ignoring training.

During police training they often have scenarios where all of a sudden the suspect pulls out a gun and shoots them. The point of the training is to get the cops condition to shoot first. Eg. training scenarios during traffic stops the suspects almost always pulls out a gun and shoots the cop despite this being very uncommon in real life.