If you look into it, you’ll realise the underlying theory quite obviously came to be when someone very smart tried to figure out how could a flat earth work without throwing all physics out of the window. It’s actually pretty neat. There are obviously details that can be tested for and the model disproven, but it does account for a lot. IIRC the basis is that the flat Earth is constantly accelerating at 1g, which provides gravity. Per theory of relativity you can accelerate at a constant rate for an arbitrary length of time, so that works. I think stuff such as phases of the Moon etc are also accounted for, though I don’t remember the details of that.
Really, people get too caught up in finding holes in the model when the most obvious flaw is that the whole thing requires tens of thousand people at least all knowingly covering this up without getting anything out of it. But if you look at it as a thought experiment, not an attempt to describe the reality, you’ll find that it’s really pretty cool. Or, it was cool, before idiots started to actually believe it.