Because of the quantity of harm that can be caused. We let people own kitchen knives. (Most) countries don't let people own semiautomatic rifles and drum magazi

Because of the quantity of harm that can be caused. We let people own kitchen knives. (Most) countries don't let people own semiautomatic rifles and drum magazines. No country allows people to own VX gas grenades or multiple grams of Cobalt-60. It's one thing if one asswipe decides to engage in malice by themselves. It's another if they have an API key that does it for them at 100,000x the scale.