Because, believe it not, most cultures have experienced memorable local floods at some point.
It happened just as stated.
"As stated" by *which* culture? most of the various flood stories differ greatly in their details. And if there had been a worldwide flood within recent history, the scattered stories of it should be far more consistent than they are.
Furthermore there would be unmistakable, clear evidence of it in dozens of different independent scientific fields. And there isn't. Additionally, there is clear evidence that there *hasn't* been (due to the existence of patterns of evidence which would not be found as they are if an Earth-submerging flood had happened within the past 500 million years).
Consider the fossil record: billions of dead things buried in sedimentation ("laid-down-by-water rock") found all over the earth. Geologist Dr. John Morris explains, "Sedimentary rocks, by definition, are laid down as sediments by moving fluids, are made up of pieces of rock or other material which existed somewhere else, and were eroded or dissolved and redeposited in their present location." [1] Over 70% of the earth's surface rock is sedimentary rock (the rest of earth's surface rock is volcanic igneous and metamorphic rock). In these sedimentary rock layers, geologists find some very odd features. For example, fossilized trees buried at all angles, upside-down and right-side-up, often passing through multiple rock layers, obviously the result of a marine cataclysm. These "polystrate" fossils (poly, meaning more than one; strate, meaning rock layer) are a worldwide phenomenon.There's plenty of discussion out there on scientific support for the biblical account of the flood if you're interested in reading up on it. It really is quite fascinating, and quite intellectually satisfying for those with inquisitive and scientific minds.Consider the ratios of dead things we find buried in this sedimentary rock: "95% of all fossils are marine invertebrates, particularly shellfish. Of the remaining 5%, 95% are algae and plant fossils (4.74%). 95% of the remaining 0.25% consists of the other invertebrates, including insects (0.2375%). The remaining 0.0125% includes all vertebrates, mostly fish. 95% of the few land vertebrates consist of less than one bone. (For example, only about 1,200 dinosaur skeletons have been found.)" [2]
Also consider the abundant fossil remains of marine life found atop every mountain range in the world. For example, clusters of hundreds of gigantic (300kg/650lbs) oysters found atop the Andes Mountains in South America. [3]