Although an elusive bunch, Fairies exist within every dimension as we do. As plants, our animal counterparts, Star People, Dragons etc. Being derived from divine sources ( although many stories reek of their "evil havoc" which can't be trusted because they are out of fear and ignorance of not knowing.) These spirits can be thought of as genius children. They live life just like a human. Although in many ways they are very different. Approaching things objectively on some level.
Being made of energy ( non-material form of various complex information based substances) they can be seen not by the naked eyes, but the third eye which is connected highly to our soul. Souls are the spark linking together everyone no matter what beliefs are.
They do not follow a religious belief, as religion is in fact created by humans to understand worlds.
Changeling and baby switching stories were fear and ignorance, as well as created for the purpose of understanding deformities. Time depending on your perceptions does not exist in the Fairie Lands, or is very different.
Although with Earth is an illusion. Aging is required to learn, grow and such. Otherwise, we would remain babies.
Because the Fairy Land is so perfect, the food is so energetically satisfying, warm happy and the fact you can create anything no human wants to leave.
This is why humans are not meant to be there.
If we were, we would be born there.