I can think of one example of why it might be hard to believe the earth is round.
If you were a corn or wheat farmer in Kansas, had never been to school and had never been more than a few miles from your farm, you might believe the earth was flat and it would be hard for you to believe the earth is round.
No offense intended to the farmers in Kansas. Where would we be without all the wheat and corn they produce ?
And I am sure most of them are reasonably intelligent.
It would be interesting to hear their side of the story.
I have relatives who are farmers in Kansas as well as others who are land levelers. They use lasers and GPS systems to plant their crops with incredible precision, and to level the land in such a way that at any given point water will act as though it is on level ground. In order to do this they know very well that they need to account for the curvature of the earth when working over long distances. I am not aware that they have had any issues with light refraction but I am going to ask them about that the next chance I get.
I think that example would be better if you considered the Kansas farmer of the 1900's to the 1950's rather than 2016. There would be more chance then about him believing the earth was flat and finding it was hard for him to believe the earth was round.
For example, one of my favorite "picture books" is "U.S. 40 . Cross Section Of The United States Of America", by George R. Stewart. If you did not know better, just looking at the pictures he took in Kansas might lead you to find it easy to believe that the earth was flat and hard to believe that the earth was round. However, the pictures would dispel any doubts about the horizon being a distinct line, the distance to the horizon depending on the height of the photographer above the ground and not some imaginary flat earth idea that you would see "an indistinct blur which fades away in an indefinite distance." Mr. Stewart was very fortunate in having very clear weather when he took his pictures and they are very sharp, especially in showing the horizon.