Because they're cheap and easy to make. I mean, they're made out of paper and you unfold them from the party decorations and wear them. Especially the person having the birthday, because that one should be bigger so everyone can tell who the birthday party is for.

It's just a tradition of birthday parties on Earth. What do you mean you were made by spacers? Were you not born like most people? Do spacers make clone babies or something? Are you a clone baby?