Because iPhones are not layers of glass glued together from either end, with a battery sandwiched in-between. Apple gets a lot of shit (deservedly or not), but they have taken pains to make refreshing their phones (battery, screen, etc) a not-garbage experience

If you want to replace a battery on a relatively modern Samsung flagship, you will likely swear, drink, and/or curse the engineers that decided "Glass and glue is good enough"