it's pretty rare for an ordinary person to have the opportunity to directly observe the curvature of the earth. I personally don't notice it when I fly commercially. you can indirectly observe it with binoculars on the beach by watching ships (dis)appear over the horizon, but a) you have to recognize the implication, and b) this can be confounded by a mirage/shimmer effect.

it's not hard for me to imagine that some extremely skeptical people might doubt that the world isn't simply how it looks: flat.