A common fallacy of Flat Earthers is to say "if the earth is round why does it look flat to me?" The problem with this fallacy is that they assume that standing on a flat earth would look like what we experience every day.
But the truth is, if we lived on a flat earth, our horizons would look completely different. On a flat earth, our horizons would always have some mountain range, hills, or city off in the distance. For instance, if you were standing anywhere in Kansas (probably the flatest state in the Union), the Rocky Mountains would be visible in the West and the Appalachian Mountains visible in the east.
To the south we would see Oklohoma City. It would be small but it would still be visible.
If we stood at the edge of the ocean, we could see across the ocean to the opposing coastline. And if we travelled across the ocean, both coastlines would never leave our view.
That's what our horizons on flat earth would really look like.