Action and reaction pairs of forces always act simultaneously, are equal in magnitude and opposite in direction, and act on different systems, which is why they do not cancel each other out.
Action and reaction pairs of forces, according to Newton's third law of motion, always act simultaneously. They are equal in magnitude but opposite in direction, and are exerted on different objects or systems. This means that the force exerted by object A on object B is the action, while the force exerted by object B on object A is the reaction. These forces do not cancel out because they are acting on different systems, and choosing the system of interest is crucial to analyzing a problem in physics.