When you look out of an airplane window, it's true that the Earth's curvature is not always immediately apparent. However, there are a few reasons why the curve may not be easily visible from a typical cruising altitude.
Scale: The Earth is indeed a large sphere, but it's also enormous compared to our everyday surroundings. When you're in an airplane, the height at which you're flying is relatively low compared to the Earth's overall size. As a result, the curve of the Earth's surface appears quite gradual and subtle.
Altitude: Commercial airplanes generally fly at altitudes of around 30,000 to 40,000 feet (9,000 to 12,000 meters). At this height, the Earth's curvature is still there, but it may not be visually striking. You need to be at a much higher altitude, such as in the upper atmosphere or in space, to see a more pronounced curvature.
Field of view: The window size of an airplane is relatively small, and your field of view is limited. This restricted view makes it challenging to perceive the curvature of the Earth fully. Additionally, airplane windows are often curved themselves, which can distort the perception of the outside world.
Atmospheric distortion: The Earth's atmosphere can cause optical illusions that affect our perception of the curvature. Factors like atmospheric haze, the angle of sunlight, and the density of the air can impact the visibility of the curve, making it harder to discern.
However, there are situations where the Earth's curvature becomes more apparent. For instance, when viewing the horizon from higher altitudes, such as in certain flight maneuvers or when observing from the windows of high-altitude aircraft like the Concorde or specialized research planes, the curvature may be more visible.
Moreover, if you observe photographs taken from space or at very high altitudes, such as those captured by astronauts aboard the International Space Station (ISS) or from satellites, you can clearly see the Earth's curvature.