The perception of curvature at the horizon depends on the observer's height above the Earth's surface and the extent of their field of view. While Earth is indeed a spherical shape, the curvature is not always readily apparent from ground level due to the large scale of the planet.
On a small scale, such as standing on a beach or looking across a relatively short distance, the curvature may not be noticeable to the naked eye. However, if you were to observe the horizon from a higher vantage point, such as a tall building, a mountain, or an airplane, the curvature becomes more apparent.
The reason for this lack of perceived curvature at ground level is primarily due to the Earth's size. Earth has a diameter of approximately 12,742 kilometers (7,918 miles), which means that the curvature of the planet's surface is relatively gentle over short distances. The horizon appears flat because the curvature of the Earth is not immediately discernible within the limited field of view available to us at ground level.
Additionally, atmospheric conditions, such as haze, fog, or other visual obstructions, can further hinder the visibility of the curvature. These factors contribute to the common observation that the horizon appears flat when looking out over large bodies of water or open landscapes.
However, it is important to note that there is ample scientific evidence and various visual demonstrations that prove the Earth's curvature, such as photographs taken from space, satellite imagery, and the ability to observe ships disappearing hull-first beyond the horizon. The lack of perceived curvature at ground level is simply a result of the Earth's vast size and the limitations of our human perception.