It is often said that, if the Earth were flat, we could see all over it! This is the result of ignorance. If we stand on the level surface a plain or a prairie, and take notice, we shall find that the horizon is formed at about three miles all around us: that is, the ground appears to rise up until, at that distance, it seems on a level with the eye-line or line of sight. Consequently, objects no higher than we stand - say, six feet - and which are at that distance (three miles), have reached the "vanishing point," and are beyond the sphere of our unaided vision. This is the reason why the hull of a ship disappears (in going away from us) before the sails; and, instead of there being about it the faintest shadow of evidence of the, Earth's rotundity, it is a clear proof that Earth is not a globe.
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.