Why can we not see the whole earth at a time?

We cannot see the whole Earth at once due to its spherical shape, which means that only half of the surface is visible from any given point in space. Additionally, Earth's curvature limits our line of sight, as the horizon blocks the view of areas beyond it. Even from high altitudes, the size of the Earth means that a complete view requires being far enough away, such as from space, to encompass the entire planet.