I will agree that they probably had some elements within them which were not completely fabricated. Like any good fiction, blending some reality into the story enhances it, making it easier to accept it as possibly real. Whether or not the primary point of the story is real depends on the particular story.
The catastrophic flood stories, for example. Most early civilizations were centered upon large rivers or natural harbors, areas which would tend to see major flooding periodically, and truly catastrophic flooding rarely. For commoners who rarely, if ever, strayed more than a day's journey from home, seeing everything they've ever known covered in a flood would certainly engender tales of the wrath of the gods destroying the whole world. In effect, the world they knew was destroyed. Not a hard concept to understand. Taking those stories and twisting them into some sort of morality story creates the myth. An angry god: what was he angry about? (People sinned, or they didn't pray hard enough, or they didn't sacrifice enough virgins.) The world destroyed: why would he do that? (To punish everyone, guilty and innocent alike, men, women, children, even animals, except for one righteous family.) Control: how do we make sure he doesn't do that again? (Don't sin, pray harder, sacrifice more virgins. And don't forget to pay the priests.)
So a tiny incident (globally speaking) is blown up into a major myth. Floods happen. Global floods don't.