Imagine an ant walking along the surface of an orange, into your field of view. If you look at the orange “head on”, you will see the ant’s body slowly rising up from the “horizon” because of the curvature of the orange. If you would do that experiment with the ant approaching along a long road rather than a round object, the effect would change: The ant would slowly "materialize" into view (depending on how sharp your vision is).
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.