Here's why:

* Earth's Curvature: The Earth is a sphere, and the curvature of the Earth is what creates the horizon.

* Perspective: Our eyes are positioned on the surface of the Earth, and we see a straight line where the Earth's surface meets the sky.

* Distance: The horizon appears to be a straight line because it's so far away. The curvature of the Earth becomes apparent over much longer distances.

However, there is a slight curve to the horizon:

* Atmospheric Refraction: The Earth's atmosphere bends light, making objects appear higher than they actually are. This effect creates a very subtle upward curve to the horizon, especially at sunrise and sunset.

In summary:

The horizon at sea level appears as a straight line due to the curvature of the Earth and our perspective. There is a very subtle, nearly imperceptible curve due to atmospheric refraction.