This is often phrased as “shoot until the threat is terminated.” But, in the real world, that advice actually means “shoot until you can tell that the threat is terminated.” Police aren’t psychic.
Modern pistols allow the user to fire multiple shots per second.** Deciding that a threat is “terminated” takes time. Put those two things together and – if police continue to follow this rule – you have a recipe for every target to be shot many, many times.
**(Not-so-fun fact: The record with a revolver appears to be 8 shots in one second.)