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).