They literally just stick a battery on the back of a case. I'm sure they could modify the design of the battery so it fits the entire length of the case.
They do it this way for a couple reasons. Keep the cell signal strong. Funny thing about most battery cases is that they hurt cell signal and make the phone work harder to keep a good signal, using more battery. Probably also allows them to help kee the phones from throttling due to high temps.