Li-ion cells are available in AA- and AAA-scaled sizes, e.g. 14500, 10440, plus fractional sizes that can be shorter or longer for the same diameter. If we wanted to not glue batteries into devices, we very well could, but that would make it harder to force purchase of a new device when the consumable component inside it fails.
I think very few manufacturers are optimizing for that. The move to integrated batteries for most portable electronics happened when the price of the battery plus charging ICs became lower than putting in a battery holder. Doing battery holder is currently simply more expensive, design is more complex putting it together is more complex. The cost are not intuitive, you can get 10+ microcontrollers for a price of a single physical on-off switch.
