If you were to mount a pod filter inside the engine bay, in the same spot as the factory air box, the filter would suck in hot air from the engine (particularly the heat coming off the exhaust).
This is why you should make up a Cold Air Intake, because they bring the air in from the front of the car, which is much cooler than the air that is in the engine bay.
A Cold Air Intake doesn't actually make the air entering the engine any cooler than it is outside the car.