Because it is spherical. If you know what you are looking, you can see with your own eyes at calm sea that there is something blocking the eye sight of a, for example, island shore, when you look at it from, for example, 3 miles, eyes on hight of one and a half feet. But, suddenly, shore can be seen if you climb just three feet... That thingie blocking the sight is called curvature of the Earth. I saw that with my own eyes. No question about it.