External batteries are not the same as there is substantial loss in transmitting the power to the phone, particularly with the many “magsafe” compatible wireless ones. The wired ones add substantially more bulk for similar battery size and although the standard for battery life is much better now, for many otherwise great phones it’s still not amazing (aka every pixel prior to this year’s).

Being able to quickly swap a battery or simply replace it with a 10000mAh cell for only a few mm more thickness (my preferred method) simply isn’t an option now.