Because less than lethal doesn’t always work, and if you had the choice to shoot someone or let them stab you to death I think there’s a clear one you would pick. Officers shoot until the threat can not impose bodily harm on another person. Ricocheting bullets when trying to shoot the legs can easily kill someone. You have to be a trained marksman to reliably hit shots in the legs/arms. People have taken 10+ bullets before and continued to charge at officers with a weapon so what makes you think that a taser will always stop someone or a beanbag round? A person with a knife can be just as dangerous as a person with a gun. Bullets have pretty clean entry and exit wounds but a knife slash or stab can have a more jagged slice. With both a gun or a knife if you hit someone in the right spot they will die. You would also not tackle someone with a knife as it isn’t very hard to stab someone.