Because humans that’s why.
It’s funny to think that an omnipotent god in all his infinite wisdom would choose among all options available to him, the very error-prone and inhomogenous method of literature to spread his world to his people, most of whom at that time couldn’t even read. 
BUT if humans invented scripture, which they did, then the most efficient way to spread it would be through literature.
So, again, Because Humans. That’s why