What myths are

Stories that do work.

Function

Myths explain how the world began, justify customs and hierarchies, encode practical knowledge, and give shape to fear of death. They were rarely idle entertainment.

For their communities these were often sacred narratives, not fiction.
The word myth in everyday use meaning falsehood is a modern development.

Transmission

Most were oral for generations before being written down, often by outsiders or by later converts, which shapes what survives and how it reads.