Where did all these stories come from? There are dozens of them from people groups all around the world. From Scandinavia, Egypt, East Africa, Korea, Australia, Pacific islands, Native Americans, and many, many more. See Mark Isaac's flood story collection.
There are three major explanations....
The first is that all the world's flood stories are mere tales, evidence of a common "flood" gene yet to be found in all human beings, that expressed itself from time to time by certain carriers of that gene to create entertaining stories centered on a flood motif (perhaps I am one of those carriers?!). By the year 2100, PhD Evolutionists will apply for grant funding, and will then make a press release that announces that they have isolated this gene. They will then do studies to show how this gene links to other tendencies within human beings, like an irrational preference for prayer to a higher being, for example. (I will make myself available for the brain chemistry study...)
The second is that the most ancient flood story of all is actually true (couldn't be Noah's flood from the Bible, could it?!), and somehow made a vast impression on all peoples across the entire world (couldn't be because the flood was global, could it?!). All the other stories multiplied and got garbled for some reason (couldn't be the Bible's Tower of Babel, could it?!).
The third is that one of the other flood stories is the most ancient one, and is the one that is actually true. Perhaps it is the Native American Lakota story that tells us that Unktehi, a water monster, fought the people and caused a great flood. A better candidate might be from East Africa (in the area where humans first evolved from their apelike forebears) - the Masai story of Tumbainot, who built an ark of wood and enter it with his two wives, six sons and their wives, and some of animals of every sort.
Which explanation do you think is correct?