Because if they did either, the size of the batteries and/or the access panels and mechanisms required to access the battery would make the unit significantly larger than it is, likely by several millimeters in thickness at a minimum, and would also affect other dimensions, as well as weight. It was an engineering decision to use an integrated battery; if it were not integrated, the unit would not have the small, sleek form factor that makes it so attractive in the first place. Additionally, the iPod's battery is indeed replaceable.