Any sphere appears flat when you zoom in close enough. People who use this line of reasoning to support their belief in a flat Earth simply do no understand magnitude and scale. At sea level, a 6 foot tall person will see at most, 0.024% of Earth's curvature. Any curve will appear flat if enough magnification is applied. At an airliner's cruising altitude, you will see at most, 1.0% of the curvature, but only from the cockpit. From a passenger window, you will see about 1/3 of that. The Earth is incredibly large when compared to smaller objects that a person is innately familiar with. Further, the curvature of the Earth can occasionally be seen when very high up. In fact, "on a clear day, the curvature of the Earth can be seen from an aeroplane window."
Additionally, the horizon does NOT "always rise to eye level." Many people have performed simple experiments to prove in order to prove this line of reasoning. ^{[5]} ^{[6]} One method is to draw a circle on a piece of paper. Then cut out 1% piece of the circle, and notice how little curvature of the circle's perimeter you can see. This is why it is so difficult to notice Earth's curvature from the surface.