Haze

Air is not perfectly transparent. Over short distances you don’t notice anything, but over long distances you’re looking through more and more air, and it becomes harder to make things out clearly due to the water vapour and the like in the medium you’re looking through.

This is why we cannot see on endlessly. Even if there were no obstacles between you and an object in the distance, even though the Earth is flat you couldn’t see it through the accumulated haze.

The impact of atmospheric haze is greater at ground level, however, as air is thinner at higher altitudes. As such, if you want to see further (or be seen from further away) it is beneficial to ascend. The higher you go the thinner the air and the atmospheric haze are, and so the easier it is to see or be seen.

Similarly, if you look up you can see the more distant Sun, moon and stars despite how far away they are because the haze is much thinner above you. At sea level though, haze can obscure far more easily.