Because a narcotic by definition is slowing down the human brain. Anything that messes with the human brain is dangerous. Also, the argument that it is less dangerous than cigarettes or alcohol is not that convincing, seeing as (manufactured) cigarettes come with jet fuel, battery acid, tar, and menthol, while alcohol literally poisons the liver. All natural tobacco cigarettes are not as bad for you as weed btw. I'm not going to go and say it should be illegal, because that would just lead to a larger black market, but I do believe the world would be less violent and more productive without narcotics such as these. However, its your own choice what you put in your own body. (Obviously with marijuana there are some exceptions, such as the products that use it after the drug part had been removed, and when it is medicinal in nature, as drugs for pain need to be narcotic in nature when the pain is from anything other than swelling).