My own addendum to the theory is that this is why they've introduced MagSafe charging – if you have a power bank in your bag that is supposed to be charging your phone, but its wireless charging its just gonna fall off / come away from the charger. With magnetic attachment that is still more likely than an actual cable, but much less so.
Power-bank usage is clutch in professions that involve both a lot of phone communication and movement. Event staff an politician's offices (which have a surprising amount of overlap) both will run on multiple power banks a day during "Go" time.
Maybe in a few years wireless charging will be efficient enough that you don't have to bring a larger power bank to compensate for power loss.