Nativity Scene

I got a new Nativity Scene this year. When I was little I always wanted to play with the one my mom had, but it was a hand carved beauty from Isreal. My new set is a more durable model.
It really is darling. I read one discussion on PYP about Christmas Traditions. One gal said that her family only puts the nativity under the tree until Christmas Eve. That helps her focus on the true meaning of Christmas. I thought this was a great idea. The only problem is that "organizing" the Christmas presents under the tree was always my favorite. I would sit on the floor in the tree lit living room and spend hours arranging the presents so they looked just right. I'll just have to do it on Christmas Eve.
My only question is, why do they have to sell the shepherds seperate?