On a Flat Earth, the horizon 'keeps going', allowing you to see as far as the atmosphere allows. On a Round Earth, the curvature would inhibit your sight so standing in a tree, on your roof, in a sky scraper, in an airplane, or out in space would let you see more. Guess what. It does! The more drastic the height, the more apparent the effects of seeing further. On a Flat Earth, You would see further at ground level because it is a straight line of sight. Going higher on this model would create a longer line of sight (the hypotenuse of the triangle). Thus more atmosphere interference seeing less. We in fact see just the opposite.