What is one essential element in the story of echo that makes it a myth?

The story of Echo makes it a myth because it seeks to explain a natural phenomenon. 

The story of Echo explains why sounds echo. Echo was cursed to repeat on the last word someone had just spoken. This story explains the natural phenomenon of the echo -- because Ancient Greeks would not have understood what causes sounds to echo, the myth of Echo explains it.

That’s nice! sounds really fun.