The most commonly accepted explanation of this is that the space agencies of the world are involved in a conspiracy faking space travel and exploration. This likely began during the Cold War's 'Space Race', in which the USSR and USA were obsessed with beating each other into space to the point that each faked their accomplishments in an attempt to keep pace with the other's supposed achievements. Since the end of the Cold War, however, the conspiracy is most likely motivated by greed rather than political gains, and using only some of their funding to continue to fake space travel saves a lot of money to embezzle for themselves.
20.) The common sense of man tells him – if nothing else told him – that there is an "up" and a "down" in -nature, even as regards the heavens and the earth; but the theory of modern astronomers necessitates the conclusion that there is not: therefore, 'the theory of the astronomers is opposed to common sense – yes, and to inspiration – and this is a common sense proof that the Earth is not a globe.

156) People also claim to see curvature in Go Pro or other high altitude camera footage of the horizon. While it is true that the horizon often appears convex in such footage, it just as often appears concave or flat depending on the tilt/movement of the camera. The effect is simply a distortion due to wide-angle lenses. In lens-corrected and footage taken without wide-angle technology, all amateur high-altitude horizon shots appear perfectly flat.
As for flight paths and what Appears to be the silly way for a ball earth but makes sense for a flat earth, it reminds me of the child quiz. There is a spider in the corner of the room on the floor and he wants to get to the opp corner on the ceiling. Which is the quickest path? We instantly say, across the floor and up the wall join. BUT, if we flatten the room we then draw a straight line, we find the quickest path is diagonally up one wall and then diagonally across the ceiling, which Looks longer but is best.
There is no problem more important to the astronomer than that of the Sun's distance from the Earth. Every change in the estimate changes everything. NOW, since modern astronomers, in their estimate of this distance, have gone all the way along the line of figures from three millions of miles to a hundred and four millions - today, the distance being something over 91,000,000; it matters not how much: for, not many years ago, Mr. Hind gave the distance, "accurately," as 95,370,000! - it follows that they don't know, and that it is foolish for anyone to expect that they ever will know, the Sun's distance! And since all this speculation and absurdity is caused by the primary assumption that Earth is a wandering, heavenly body, and is all swept away by a knowledge of the fact that Earth is a, plane, it is a clear proof that Earth is not a globe.
Then God created land and vast oceans. The oceans were expansive and deep, with sea life, but the Bible also describes another body of water called the “deep”. Is it just describing all the water on earth, or is there something else, like a deep denser ocean beneath Earth? In Genesis 6-7, God causes a flood to destroy the earth because of its wickedness. The flood waters came from the “deep” below the earth and the waters from above the heavens (the firmament. In Genesis 7:11 it says, “In the six hundredth year of Noah’s life, in the second month, the seventeenth day of the month, the same day were all the fountains of the great deep broken up, and the windows of heaven were opened.”

As the mariners' compass points north and south at one time, and as the North, to which it is attracted is that part of the Earth situated where the North Star is in the zenith, it follows that there is no south "point" or "pole" but that, while the centre is North, a vast circumference must be South in its whole extent. This is a proof that the Earth is not a globe.
111) Since the North Pole and Antarctica are covered in ice and guarded “no-fly” zones, no ships or planes have ever been known to circumnavigate the Earth in North/South directions. The only kind of circumnavigation which could not happen on a flat-Earth is North/Southbound, which is likely the very reason for the heavily-enforced flight restrictions. The fact that there has yet to be a single verified North/South circumnavigation of Earth serves as standing proof the world is not a ball.
103) There are several constellations which can be seen from far greater distances over the face of the Earth than should be possible if the world were a rotating, revolving, wobbling ball. For instance, Ursa Major, very close to Polaris, can be seen from 90 degrees North latitude (the North Pole) all the way down to 30 degrees South latitude. For this to be possible on a ball-Earth the Southern observers would have to be seeing through hundreds or thousands of miles of bulging Earth to the Northern sky.
If we move away from an elevated object on or over a plain or a prairie, the height of the object will apparently diminish as we do so. Now, that which is sufficient to produce this effect on a small scale is sufficient on a large one; and traveling away from an elevated object, no matter how far will cause the appearance in question - the lowering of the object. Our modern theoretical astronomers, however, in the case of the apparent lowering of the North Star as we travel southward, assert that it is evidence that the Earth is globular! But as it is clear that an appearance which is fully, accounted for on the basis of known facts cannot be permitted to figure as evidence in favor of that which is only a supposition, it follows that we rightfully order it to stand down, and make way for a proof that the Earth is not a globe.

9.) As mariners take to sea with them charts constructed as though the sea were a level surface, however these charts may err as to the true form of this level surface taken as a whole, it is clear, as they find them answer their purpose tolerably well – and only tolerably for many ships are wrecked owing to the error of which we speak – that the surface of the sea is as it is taken to be, whether the captain of the ship "supposes" the Earth to be a globe or anything else. Thus, then, we draw, from the common system of "plane sailing," a practical proof that Earth is not a globe.

As previously mentioned, the reaction of bodies of water with sunlight is very different from that of land. Being largely transparent, light penetrates deeply into water, so that the sun’s light is absorbed throughout a thick layer from the surface to some depth rather than just on the surface, as with land. Additionally, water has a high specific heat, which means that its temperature increases very slowly as heat is added. Consequently, water exposed to sunlight does not change temperature appreciably throughout the day, so there is no heating of air in contact with the water. If anything, during summer afternoons, when land is rapidly heating, bodies of water frequently are cooler than air temperature. The cooler water chills the air in direct contact with it, so the air lying just above water often is cooler than air higher up. Since air temperature normally decreases with height, this temperature reversal from the norm is called a temperature inversion. Temperature inversions are common over bodies of water during late spring and into summer. Since this temperature structure is the reverse of what causes inferior mirages, inferior mirages are far less commonly noticed over water. This happens particularly during the summer, when inferior mirages are common over land.