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.

83.) It has been shown that the meridians are, necessarily, straight lines; and that it is impossible to travel round the Earth in a north or south direction: from which it follows that, in the general acceptation of the word "degree" – the 360th – part of a circle – meridians have no degrees: for no one knows anything of a meridian circle or semicircle, to be thus divided. But astronomers speak of degrees of latitude in the same sense as those of longitude. This, then, is done by assuming that to be true which is not true. Zetetic philosophy does not involve this necessity. This proves that the basis of this philosophy is a sound one, and, in short, is a proof that the Earth is not a globe.
78) From Anchorage, Alaska at an elevation of 102 feet, on clear days Mount Foraker can be seen with the naked eye 120 miles away. If Earth were a ball 25,000 miles in circumference, Mount Foraker’s 17,400 summit should be leaning back away from the observer covered by 7,719 feet of curved Earth. In reality, however, the entire mountain can be quite easily seen standing straight from base to summit.