At the time any of those myths would have originated, nobody really had any concept of a "global" anything. If the local river flooded, that was your _entire w

At the time any of those myths would have originated, nobody really had any concept of a "global" anything. If the local river flooded, that was your _entire world_. Lots of local flood stories, then once the bronze age got going and trade routes and cities and writing, they started getting coalesced into more regional myths.