I assume this is because the thermal containment bags they have are only rated to a certain limit, but distinct batteries hopefully won't ignite at the same time and can in any case go in different bags?
Heat a lithium secondary cell too much and it goes into runaway. When the cells are packed together in one device if one of them goes up it can take the others with it. The bigger the device the hotter it will be and harder to avoid it turning into a disaster.
