I suggest you read lots of legends if you think they are consistent. A few flood stories, cherry picked for consistency, do not make consistency. Bear in mind that there was significant trade between the middle east and asia.
Now if you want to speculate on an ancient worldwide flood that actually happened just a millennium or so before the earliest known writing, try the melting of the glaciers at the end of the ice age. Sea levels rising 200 feet, possibly flooding everything coastal.
We're going around in circles. The point is, the flood stories could be, at least partially, a memory of a disaster that resulted in the genetic bottleneck.
I'm not saying it is or isn't. I'm saying it's plausible.