There are many possible reasons. I’m not a flat-earther, by the way. But, I find it very interesting. So, one reason could be that the Earth is an infinite flat plane, and the elites don’t want us finding out that we can escape their control by leaving. Another is that globe-earth fits into a worldview system that reduces humanity to a random happening on a random rock flying through infinite nothingness with no destination or purpose (other ideas that flat-earthers would say are false - and a part of this worldview - would be things like abiogenesis, evolution, heliocentrism, and, for some, even atheism).