Because potatoes (like tomatoes), are a member of the nightshade family. Potatoes produce Solanine in their stalks and leaves, but can also produce the naturally occurring insecticide in their tubers, mostly in the skins. Solanine, as 'nightshade' implies, is poisonous to humans and animals. If potatoes have a greenish appearance or have sprouted, it's recommended that they be discarded or at a minimum peeled thickly and the eyes thoroughly dug out before eating, as solanine is not destroyed by cooking. The green in a potato skin is chlorophyll, an indication that the potato has been exposed to sunlight, which can also stimulate solanine production. Risk of solanine poisoning is why nobody would regard tomatoes as anything but ornamental fruits for generations. Peeling potatoes is generally a sensible practice if there's no firsthand knowledge of the food source. There's a wide variety of potatoes, and newer varieties may make less solanine than varieties of old. Potato skins can be 'normal' for a long time, but ordinarily there's no way for the consumer to know how long it's been since the potato on their plate has been harvested. Keep potato peelings away from pets and toddlers, animals and children weigh less than adults and are more susceptible to being sickened from ingesting solanine.