Because there is no place to shoot that will simply incapacitate everyone. The policy is generally shoot to stop the threat not to kill per se. There are many instances where a person will take many rounds in potentially fatal spots and continue like nothing has happened.
> In this free-for-all, the assailant had, in fact, been struck 14 times. Any one of six of these wounds – in the heart, right lung, left lung, liver, diaphragm, and right kidney – could have produced fatal consequences, “in time,” Gramins emphasizes.[1]
[1] https://www.policeone.com/officer-shootings/articles/why-one...