Because it is what it says. I can't put it in a more simple way... I don't see how it can be taken as a local flood.
Well, consider that the word translated as "earth" does not necessarily refer to the planet, even in modern English. It can also mean soil, dry land (as opposed to ocean) or land (as in territory, region).
In fact, the biblical writers had no idea the earth is a planet. In their day the word "planet" referred to a kind of star--the sort of star that does not remain in one place but moves from one constellation to another. (The word "planet" means "wanderer".) "Planet" was not used to refer to the satellites of stars until the 19th century. For biblical writers, a planet was always a heavenly body and the earth was definitely not a planet.
So when they wrote of the waters covering the earth, they could well have meant the land they were aware of, not the whole globe.
Why do you have a problem with this?
After all doesn't Jesus' genealogy include Noah? (For the person who thought it was just a story, not history)
1. That still does not make the flood global.
2. Genealogies can be stories too.
3. What do you mean by
"just" a story? Why, given the example of Jesus who did only what he saw his Father doing, would you say God never communicated in stories? Jesus certainly did.
There are many advantages to communicating in stories, especially in an overwhelmingly oral culture. Most ancient peoples, including the Israelites, never made a sharp distinction between story and history. That is a modern fetish and to impose it on the bible is to misread the text.
Why do creationists constantly demean non-factual literature? God uses it. Why insult it?