Look at this image - it's a tanker, 900 meters away. IRL it would be easy to see it, it would be so obvious, only a blind man would not see it.
And here it's barely visible, no contrast at all, almost blended with the sky. If you look slightly away, it is gone, while in real life you could easily see it with peripheral vision.
And this is a problem with visibility in DCS, where aircraft in close vicinity are hard to spot, not the ones 10 miles away.