When you increase your altitude you are changing your perspective lines in relation to the earth, pushing back your vanishing point. The vanishing point, beyond which no man can see, is created when his perspective lines approach each other at a certain angle smaller than the eye can see. If you increase your height you are changing your perspective lines and thus can see further before all sight is lost to the vanishing point.
Usually it is taught in art schools that the vanishing point is an infinite distance away from the observer, as so:
However, since man cannot perceive infinity due to human limitations, the perspective lines are modified and placed a finite distance away from the observer as so:
The vanishing point acts as the liming point of all vision, as all bodies beyond it are too small and squished to see with the naked eye.
The same effect is found on a 3D video game which assumes a flat surface. When you increase your altitude you can see farther because you are so much higher than everything else. Your computer's resolution is better able to see something below you than off on the horizon where the pixels are linearly squished.
When you increase your altitude on a plane you have broadened your perspective lines and have pushed your vanishing point back.