According to Susie's explanation, why are the people she sees in heaven there?

After Susie has been in heaven awhile she realizes that the other people in her heaven are people whose heavens have some of the same elements as hers does. Susie meets an Asian girl named Holly and they become friends. Susie describes heaven as a place where they are given their simplest dreams; whatever they desired, they got. What Holly and Susie really want is to grow up, but they are not able to experience that.