Probably because the recirculation actuators are shot. The default position of the actuators is everything closed except for the defrost vents and outside air intake flap. With the engine running and the ACC set to '0,' the outside air intake flap should close (recirc) giving you no air to any vent since the blower isn't running when the ACC is set to '0.'
The recirc actuators are easy to get to. Just remove the glove box liner and there they are. Then defrost vent actuator is to the left of and slightly higher than the glove box cavity.