When you are standing on the surface of the Earth, the horizon appears at eye level because your line of sight is tangent to the Earth's curved surface. This is due to the fact that light travels in straight lines and our eyes perceive it as such. The curvature of the Earth becomes more noticeable as you increase your altitude or distance from the surface.
However, the perceived curvature of the Earth depends on the observer's height and field of view. If you are at ground level, the curve may not be easily visible because the Earth's radius is large relative to your distance from the surface. As you ascend to higher altitudes, such as in an airplane or on a mountain, the curvature becomes more apparent.
To illustrate this, imagine you are standing on a beach looking out at the ocean. The line where the sky meets the water appears flat, but if you were to take a step up onto a ladder or a small hill, you would be able to see a gradual curve forming at the horizon. This demonstrates how the curvature of the Earth becomes more noticeable as you increase your elevation.
In extreme cases, such as from very high altitudes or from space, the curvature of the Earth is clearly visible. Astronauts aboard the International Space Station, for example, can see a distinct curve to the Earth's surface.
So, while the horizon may appear at eye level when you are at ground level, this is because the curvature of the Earth is not easily perceptible at that height and distance. As you ascend or increase your distance from the surface, the curvature becomes more apparent.