In this analysis, the effort the bank puts towards defending themselves is relevant. We wouldn't blame the bank for an army attacking them, but if they left the door unlocked and the neighbours kids made off with your money you very rightly would feel differently.
Which does make me wonder why we never really hear of banks being attacked and robbed in such a way? One would think they would be the most obvious targets to throw an army of criminals at.
It's pretty much the definition of a functional state that the police can gather more resources faster than any group of criminals. By the time you gather enough criminals to hold off the police for even a few minutes, most of the time, combined with the sibling's point of not that much physical money being stored at banks, there's not much money to go around to that many people.
Banks don't really physically store much money any more.
And more importantly - the police exist. If someone were to actually physically rob a bank, enormous resources would be spent trying to find and capture them, then they'd be thrown in jail.
If they could do the same thing, but also be physically located in another country while doing it, with no chance at all of going to jail... more banks would be robbed!
