The word ‘theory’ has several meanings and can be confusing. Its meaning in the scientific context can differ from that in the context of everyday communication.
The flat-Earth model survives not because the Earth is flat, but because every time a problem is found, its proponents would quickly invent an ad-hoc hypothesis to explain the problem away. In turn, if they discover another problem in one of these ad-hoc hypotheses, they would be happy to invent another ad-hoc hypothesis to explain the problem away. And so on, and so forth.
These ad-hoc hypotheses are there to save their core belief —that the Earth is flat— from being falsified.
Why do we believe in conspiracy theories? According to research, one reason is the so-called ‘psychological projection’.
‘Psychological projection’ is a phenomenon where someone denies their attitude or traits and assumes instead that others are doing so. They see their negative qualities in others, even when the negative characteristics are not present in the others being accused.