If there are 20 battery banks on board a plane, each possessed by a different person:

* Less likely to be of the same low quality

* Less likely to all go off

* Less likely that someone is doing something malicious/suspicious with it

vs. someone who has 20 power banks themselves in a bag, in which case if one of them catches fire unexpectedly, they will probably all go up at once and create a cumulative effect much more dangerous than 20 individuals.