I think it's due to many power banks being so shitty that the chance of them catching fire in normal operation is not insignificant.

From the EASA guidelines on lithium batteries (https://www.easa.europa.eu/sites/default/files/dfu/Backgroun...):

> Also, spare batteries, including power banks, should not be recharged while on board the aircraft. Additionally, power banks should not be connected or providing power to a device while on board the aircraft.