The Scotts' Reception Challenge involved making a Wedding Cake! That task was given to the chefs with the assumption that they should be able to create a wedding reception menu and plus the cake.
Lee Anne had some fancy, ginger-lotus type cake on her menu and was picked by the Scotts' partly because of her idea for the cake. I don't think Lee Anne would put it on her menu if she didn't know how to make it. All the chefs had cakes on their menus. It was a requirement of the challenge. They went with boxed cake because of lack of time/resources.
I thought it would be nice for the viewers to finally go back and get a chance to see these high end chefs actually build one. After all, everyday people make cakes from scratch all the time.