That there are fundamental differences means more than that they are not the same.
The NATO is not based on "a general idea" it is based on treaties, administration and the military of it's members and most important: The soldiers of it's members are not the ones who decide whether they come to aid or not and these soldiers are paid and their families won't have to face the post apocalyptic wasteland on their own when they die in combat.
The members of the NATO wouldn't jeopardize their well being if they help other members, but if a settlement loses one of their best man or woman along with their best gear, their well being will be in danger.
There is always the risk of a new Quincy massacre and no one wants to be the one who loses their best men before the Minutemen vanish.
People are selfish and they are even more selfish when their survival instinct kicks in.
People are even selfish if collaboration would benefit all of them It is called: https://en.wikipedia.org/wiki/Tragedy_of_the_commons
You can't even run a sports club, without a certain level of administration, if you don't want that someone rips you of.