Why are Aunts and Uncles used?

Two other questions have been asked now, one about Peter Parker and one about Dorothy Gale. Literature seems to have a number of cases where the hero of the story was raised by aunts and uncles. And to make it more bizarre, the parents are never really mentioned, nor is any back story given to explained what happened. Why are parents cut out of the picture so often? Couldn’t the parents be just as credible as caregivers?