Shamans/druids/Dehnars were people with very special physic abilities, we can't communicate with the "spiritual" world becouse we can't access to its "dimension".

I like to think about this "realms" like different frequencies with different dimensions that share the same space, they can't interact with eachother but by applying a modulation they can be shaped and they can become identical at the end they are essentially different waves.

And I think this is exactly what happen, Shamans with their own abilities or with the help of some substances change "dimension" and succeed in communicating with the spiritual realm.

In the same way we try to communicate with the spiritual world, the spirits try to communicate with us manifestating themself in our world with the most simple and natural langauge that exists...symbols and patters.

The reason our myths are so full of symbolical meaning is that this is the language the gods use to communicate with us...symbols and patters.