The bottom line is that tube drivers have stronger unions (RMT and ASLEF) and are in a stronger position than aeroplane drivers. If an airline's pilots decide to go walkabout for a day then the passengers will transfer to another airline. When tube drivers go walkabout for a day it's game over for tube travel.
I'm not defending or condoning them, that's the way it works for the greedy overpaid barstewards.