Why do you need baking soda in cookies and not powder?

This is incorrect: baking powder is typically used in cookies, not baking soda. Baking powder contains a combination of baking soda and an acid, along with a drying agent to keep it shelf-stable. When mixed with liquid, the acid in baking powder reacts with the baking soda, releasing carbon dioxide gas which causes the cookies to rise. Baking soda, on the other hand, is a single-acting leavening agent that needs an acid to create a chemical reaction and produce bubbles for leavening. In recipes, it is generally paired with an acidic ingredient such as lemon juice or buttermilk to produce the desired effect.