We just don't get it, there are so many other things you could say.
WHO WOULD YOU HAUNT? When you become a ghost, who will you haunt? Spooky Ghost Women's Fancy Dress Costume Dogs and cats are supposedly really receptive to ghosts, and it would be pretty nice to be able to hang out with puppies and kitties all day long! The style of the dress and hat is like something out of the 1950's, making it all the more believable that you are a ghost from the past, here to haunt the living for eternity. We aren't really sure why saying boo is a thing at all, to be honest. If you wanted to add to the scare you could say something scary like "Your soul is mine!" or "Your dead!", but we still just assume that ghosts would say boo.Oh well, maybe once we have died and come back as a ghost we will understand.