17. Why do so many ancient cultures have flood legends?
Because most ancient cultures sprung up on flood planes (because they are very fertile land), thus flood was a real and meaningful threat in their lives. It is not far from having floods as being a real and meaningful threat to having myths about a really big one. Most myths involve things which are meaningful (and often threatening) in the lives of the myth makers. For example there are great many myths involving people or monsters stealing children.