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.
If it weren’t for the issue of charging at 15w without Apple’s own usb-c base, I’d say that it already is sufficient for charging on the go.
https://www.reddit.com/r/apple/comments/jiul9p/magsafe_charg...
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.