The naive answer:
Other cultures borrowed stories from the Jews and Christians, but they may have written their stories down before the Jews and Christians did.
The problem with the naive answer:
We have written records of stories that are nearly identical to stories in the Bible, yet these stories were written by non-Jewish and non-Christian societies long before the Jewish and Christian versions of the stories were created.
The scholar’s answer:
Writers of the books of the Bible adopted concepts and stories from other cultures to incorporate into the Bible because the stories were popular or helped to explain something in the world.
Here are a few examples of such stories:
Much of the old testament was written while the Jews were in Babylon and much of the old testament stories resemble Babylonian stories written before the Bible stories. Much of the new testament miracles of Jesus were miracles performed previously by Greek and Roman deities.
Logically, we can conclude that Christianity and Judaism borrowed stories and ideas from other cultures to include popular customs, festivals, and rituals into the Jewish and Christian faiths. Inclusion of popular ideas helped foster adoption of these new religions, and gave the religions their own stories to pass along.
Some references for further research: