Because the chair exerts an equal and opposite force on the person
Explanation:
Usually, for every action force, there is an equivalent reaction force exerted in the opposite direction to the action force. This is known as the Third Newton Law. This is the same principle that is behind rockets. The ejection of hot exhaust released by the burning of fuel of the rocket (action force) causes an equivalent reaction force that propels the rocket forward in the opposite direction of the ejection of the exhaust gases.
