Question

Why are stories at the heart of every culture?

Answer 1

Because the stories create cultures, they propagate the assumptions and beliefs throughout the group in question.

Answer 2

Te daré una respuesta simple: 

Why are stories at the heart of every culture?
Because they are part of the past and have great importance for maintaining traditions and culture in general for future generations.

→En español diría algo como: 
Porque son parte del pasado y tienen gran importancia para mantener las tradiciones y la cultura en general para las generaciones futuras.