That is because they have such an active imagination that they feel like they are princesses. They may even act like them once in a while. They probably dress up like them, especially if they have seen them in a movie and have decided that the life of a princess is what they would like to live once in a while. The Disney Princesses made this popular a few years ago, but the princess Halloween costume will always be around in one form or another. When you get princess Halloween costume for a little girl, she probably has very specific ideas of what she wants to do and what that costume should look like.