This will not really be the answer anyone wants to hear, but because people settled around river valleys and locations that flooded. Simple as that.

The myth of Noah and the ark has been widely recognized as stemming from the Akkadian tales, found in the Epic of Atrahasis and the Epic of Gilgamesh. Before that, these tales likely stemmed from the still earlier tales told by the Sumerians about a figure named Ziusudra. These stories likely had a historical origin in devestating floods which archaeology has confirmed did sweep the area including the cities of Uruk and Shurrupak (see Bandstra).

Other flood myths were simply invented wholecloth. This is the case of the "myth of Atlantis" which is not really a myth, but actual fiction. Plato invented this story for his dialogues Critias and Timaeus and as I discussed elsewhere (here) there is no evidence to suggest it was historical in any fashion, and plenty of reasons to think that Plato just made it up.

Sometimes ancient authors were just smart and got super close to the truth. Ancient Greek writers knew of the presence of fossilized remains of sealife being found in the mountains. Not knowing how plate tectonics work, continental shift, and that the sea levels were also different in the past, they did not know how they got up there. BUT, they got really close to some good ideas. They theorized the world had once been submerged, and the deposits in the mountains were the remains of this.

Other places also regularly flooded. The Gun-Yu myth in China was likely based on a major flood event caused by the Yellow River, which had a devastating effect on the nearby people. Like the Ziusudra myths in Sumer, the historical occurrence of a damaging flood eventually led to the development of the myth.

So, despite the attempts of some pseudo-historical approaches like those of Answers in Genesis, or myth archetypalists who see universal concepts everywhere, the reality of the situation is that people just experienced things and wrote about them. Other times, people just made stuff up for dramatic or ideological effect, as with Plato. People are both creative, and then also inspired by the world around them.

Floods are just things that people around the world experience, and the answer is, really, pretty much as simple as that.

Other Sources used:

Barry L. Bandstra, Reading the Old Testament: Introduction to the Hebrew Bible (Cengage, 2008)

Jeffrey Tigay, The Evolution of the Gilgamesh Epic (Bolchazy-Carducci, 2002)

Irving Finkel, The Ark Before Noah: Decoding the Story of the Flood (Anchor, 2015)

Adrienne Mayor, The First Fossil Hunters: Paleontology in Greek and Roman Times (Princeton University Press, 2011)