The word “theory” has multiple meanings and can be ambiguous. Its meaning in a scientific context can be different from that in the context of everyday communication. Flat-Earthers erroneously assume we call it a “scientific theory” because it is still unproven. In reality, in the scientific context, a theory is commonly regarded as correct, & we call an unproven supposition a “hypothesis.”
One reason a person can believe in conspiracy theories is the psychological projection. They attempt to deny their negative attitudes by assuming that instead, others have them, even though it is not necessarily the case.
Some people believe others are in a conspiracy against them because of their moral problem: if they were in the same positions of the people they are accusing, they would do the same thing they are accusing others of doing.
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.