Why indeed.

I can see at least two reasons. One, the contacts are in the wrong order and the wires need to cross. Not impossible to do with copper foil (after all, all you need is a layer of sticky tape for isolation), but easier to do with wires, which you are better off doing anyway for reason two, you need to solder to the battery's original contacts. An adhesive copper foil does not make such a good and reliable connection, if it makes any at all.