A quote from Mr. Tom Bishop:
"I happen to see a perfectly flat horizon. Therefore, I can say for certain, that the earth is flat... Why should I believe in a Round Earth if my own senses tells me it is flat?"
Its actually quite simple. Any simple science will explain it. If you put something extremely small on another object that is much larger (for example: the proportion between a human and Earth) and you look across the horizon you will not be able to see all the way to the end of the object you are on. Even though the object may be a sphere, you would not be able to detect it because it is too large to see the curvature of it. Simple science, try it out sometime.