Phones tend to be designed by companies competent enough to design a phone. There's a skill floor required that just doesn't exist for power banks.

Another reason is that phones get replaced more frequently, whereas a power bank will be continually used essentially until failure. I only stopped using my last power bank because it puffed up like a balloon.

But yes, probably where this is all headed is that some day in-seat power will be banned so that you can only discharge and not charge your devices.

Galaxy Note 7 was a notable exception https://en.wikipedia.org/wiki/Samsung_Galaxy_Note_7#Battery_...