why do things vanish at the horizon? (flat earth)

our eyes perceive reality, and it comes with an illusion due to perspective, distances to objects and perceiver’s location. When boats seem to be swallowed by the great unknown when they approach the horizon, why do people assume that they go around a ball? there is no reason for this. Boats vanishing at the horizon does not prove any supposed curvature of earth. In fact, what we see happening is exactly happening because the surface around us is vast and flat into all directions. Understanding of perspective together with the understanding of levels such as clouds or airlines should at least make you doubt the ball theory, and start observing and researching actual reality, instead of trying to find truth in deceitful school books, graphics and animations.