Why we have flood legends

Because the last ice age ended about 12,000 years ago.  It had many consequences, including floods. The melting ice allowed ;and masses to rise on the North, and created massive ;lakes elsewhere.  When it all settled, the seal level rose.

BBC:As the ice melted, Britain went from being an ice-covered tundra within continental Europe to a forested area with a land bridge larger than Holland – Doggerland – across what now is the southern North Sea.
The melting ice had other consequences. In North America, meltwater created an enormous lake roughly the size of the Black Sea. About 8,200 years ago, the last glaciers holding the waters back retreated, causing the lake to burst and flood into the North Atlantic. Over the next 100 years, sea levels rose by 6.6-13.1 feet (2-4m), Hill says.It would have been an extremely attractive place to live: a low-lying plain characterised by lakes, estuaries, forests and salt marshes, according to maps by Gaffney and other researchers that were put together using oil companies’ seismic survey data.
Rising sea levels ultimately would drown Doggerland, giving it the nickname “Britain’s Atlantis” and making Britain an island once and for all. “This was Britain’s first Brexit, when we left the continent,” Gaffney jokes.
By the time the tsunami hit, sea level rise already had made Doggerland – and Britain – an island, most scientists agree.
But as it collided with Doggerland as well as the coasts of Norway, Scotland and North Sea islands like the Shetlands, the tsunami likely would have devastated Mesolithic coastal settlements.