Because many on the right are self righteous people that think they know more than the average person, due to the information they feed each other during their daily circle jerks. Painting the left in the same color enhances their righteousness, and fuels their illusions of patriotism in the face of constant seiges from the communist boogieman that has threatened their father's and grandfathers. Generalization replaces rationalization, labeling replaces logic, and condemnation replaces conversation.