It actually started as kind of an internet joke, satirical people claiming they'd be flat earthers just to troll.

But then a bunch of people that generally mistrust science and "official facts" felt their view confirmed by this and started a whole movement out of it. Some of them are from the heavily religious camp that say the bible should be trusted by the literal words, others are from the same group that called the moon landing fake, so kinda "universal conspiracy nuts" that question everything that some official person said.

So the people who are now in that movement kinda believed in flat earth and similar stuff long before, they just did so privately and didn't try to convince others of the "truth" online.