Why? Henbane and Jimson weed grow wild all over North America. Henbane grows in my back yard, Jimson Weed, Datura Stramonium, grows across the street in a field by the edge of the woods, and even the leaves from tomatoes contain the stramonium/Belladonna alkaloids..which is why you are not supposed to eat them. Bittersweet nighshade grows everywhere..it is that little climbing vine peope call a "weed" that has little purple and yellow blossoms that look exactly like minature eggplant blossoms. Same thing for potato leaves.
It is a pretty large class of plants.
My grandfather used to say that a weed was just a plant that no one had figured out a use for yet.