The last time I looked across the dry Texas plains, the earth appeared flat. Have you ever looked across the plains and noticed curvature?
Last time I looked up at the surrounding mountains from the valley I was standing in, the earth looked concave: therefore, it is concave.
Try harder Kathleen. Simply putting your eye close to a big ball will demonstrate how the closer you are to a gigantic curve the flatter it looks.
Exactly, land will appear flat, and seeing as it is a curve you won't see curvature on a plain, even if you can see 12 miles out, because the ground will go down, and at that distance trees/cacti would not bee too visible.
Plus land is a bad medium for showing the earth is round or flat because the ground can deviate and we won't know. Water can still deviate but usually nowhere near as much as land can, and it will be noticeable. That's why textbooks never use a car appearing as an example of a round earth, they always used a ship in water.