The existence of 'The Conspiracy' is a consequence of the FET. Virtually no one begins with 'The Conspiracy' and develops a belief in the Flat Earth Theory. Flat Earthers starts with the knowledge that the earth is flat, as they believe that all the evidence which they are personally able to collect and verify confirms this fact. As a consequence all the evidence to the contrary, much of which they are unable to personally test/verify is viewed as being false. The existence of such a huge quantity of false information indicates the existence of the conspiracy.
Essentially the reasoning boils down to -
P1) If personally unverifiable evidence contradicts an obvious truth then the evidence is fabricated
P2) The FET (Flat Earth Theory) is an obvious truth
P3) There is personally unverifiable evidence that contradicts the FET
C1) The unverifiable evidence that contradicts the FET is fabricated evidence
P4) If there is large amounts of fabricated evidence then there must be a conspiracy to fabricate it
P5) There is a large amount of fabricated evidence (see C1)
C2) There must be a conspiracy to fabricate it.