It's a theological argument held deeply in the Muslim faith, which explains how it's spreading so much in the last few decades, and also why they defend it with such fervor. When you attack claims of flat earth, you're attacking the Quran. Other things they believe is that the sky is a literal blanket covering the earth. Something about a firmament. I find it amusing that the plurality of the western flat earth believers don't even know where the conspiracy originates.