Please explain why is pneumonia not an interstitial lung disease?

Tissues involved: Interstitium is that area of the lung in between the blood vessels and the alveoli, more the connective tissue area. Pneumonia is an inflammation causing involvement of all tissues like the interstitial, the blood vessels and even the alveoli hence the whiteness seen or the infiltrate is dense and involves the whole area.

Consider an analogy: Regular pneumonia would fill the rooms in a hotel. The interstitial pneumonia would infect the walls of the rooms. When examined under the microscope the difference is clearly different. The chest x-ray is also different.