[freebeard]

That is simply not true--many, many cultures across the world and throughout history have believed in a flat earth. Post-Roman Western Europe was not one of them, aside from a few individuals (similar to today).

Okay, but I still don't know why the idea has re-emerged today.

Perhaps I should re-read Hamlet's Mill, it's been a few decades.

Hamlet's Mill: An Essay Investigating the Origins of Human Knowledge and Its Transmission Through Myth by Giorgio de Santillana and Hertha von Dechend is a nonfiction work of history and comparative mythology, particularly the subfield of archaeoastronomy.
Author:Giorgio de Santillana, Hertha von Dechend
Subject:Mythology and Astronomy