He pulls the trigger of his shotgun. This ignites the primer, burns the powder, and launches 1 and 3/8's ounce of number six shot at over 1300 feet per second into his chest a dozen feet away. That is over 600 grains of lead. Yes, they are individual pellets, but at very short range they are more like one solid object. Place a piece of paper 15 feet away and shoot it with birdshot. Then count how many holes you see.
First, they don't move like one solid object once they enter the body.
Second, imagine the person is on PCP, carrying a knife. You shoot him in the chest, destroying his heart which will cause him certain death. His brain has enough oxygen in it for 3-10 seconds. He runs up, slashes your throat, then falls down dead. You're bleeding with your throat slit next to him, likely to die yourself.
While this is a hypothetical situation, it's a possible one. I'd rather have the buckshot, blast him in the chest, hope it penetrates far enough where a pellet strikes his CNS and he becomes paralyzed and dies.
People aren't saying birdshot won't kill anyone...it will. It could disrupt the CNS and kill someone instantly too. What we're trying to say is buckshot has a better chance of doing this than birdshot does.
Why take chances with your life or the life of your family members.