Basically because its deeply oversimplified to the point of being nonsense. As it stands, some assholes in suits set the price. There are constant bouts of shrinkflation, some stores raise prices based on brand prestige, convenience, friction, or because a neighborhood is wealthy. Economics classes create a distorted view of how the world works. And that narrative they taught us is designed to help us accept neoliberalism. And disregard alternatives.