In order to believe that the earth is flat, you have to believe that maps of the earth are a lie. This includes the globe model, which is the widely-accepted model of the earth as viewed from space.
The flat earthers subscribe to the Gleason Map. This map shows what the
world would look like if it was flat. It's basically a flattened-out globe, which is why so many flat earth people find it easy to believe that this map is the case. It's not too far from what the "myth" is, which makes it seem like it could be the truth.
Flat earthers aren't completely wrong here. Maps are actually misleading. The size of the northern hemisphere has been significantly amplified. This is, of course, because the northern hemisphere is where the power lies. The map creators fudged the scales in order to make the countries of the north seem larger. This doesn't mean that the entire model is a lie, though.