Because the standard field of view in the game displays a panorama image up to approx. 120° (not shure about the exact value but you can adjust it in the settings). The average human field of view is at ca. 180°. If you put this panorama image on your monitor things get smaller than you would usually notice them. So if you zoom in, you get a field of view of about 30° (?), which ist approx. the angle between your eyes and the edges of your monitor and you see things in the same size you would do in real life at the same distance. This feature helps to maintain both a good situational awareness and recognize things at greater distances as you would do it naturally. Hope that made it clear though I wouldn't rely on the numbers above.
