It doesn't matter if you're trying to impress some desert rat or proving your independence over that jerk who won't let you use the army, some days you just want to look pretty. And face it, in this country Disney INVENTED the concept of royalty!
Yes, fine, our nation was founded on the right of the individual to better themselves through hard work instead of via a miracle of genetics. But to a little kid, that's BORING! So it's either let 'em play princess or teach them how to dump tea in the neighbor's bathtub.