[1dontnkow_]

Because most other countries don't even have a quarter of the military and resources the US has and there its usually about survival and defense of their borders with direct neighbors or at worst case region.

But with the US military, at least the years I was alive, it seemed to be never a real national security threat at the borders but very far away and for so not obvious reasons.

In the end however I agree with your "defense is necessary" and that's why NATO exists and should be developed further so its more strict and almost all nations worldwide join.