Or it could simply be that humans are emotional, and they want to believe they have some kind of control over the world, so they make gods, they make stories for those gods, and then they draw the pictures, tell the stories.
When Christianity came along, they couldn't go "here's something totally new", they just adapted what already existed, to make adaption easier.
The flood story of Noah isn't just in the Bible.
en.wikipedia.org
"
Flood myths are common across a wide range of cultures, extending back into
Bronze Age and
Neolithic prehistory. These accounts depict a flood, sometimes global in scale, usually sent by a
deity or deities to destroy
civilization as an act of
divine retribution."
Clearly in the modern era we can see floods for what they are. Imagine you lived in a place and everything you could see was flooded, you might imagine the whole world has flooded.
Monotheism existed a long time before Christianity, coming from Egypt and was born and died and born again, and passed through Greece to Rome and onwards to the rest of Europe.
Many of the stories existed before.