I do not know if the same "logic" holds true for others, especially the larger nations, but to many in my nation it's "if we leave there's no telling who of the neighbors is going to absorb us". Perhaps going by the "there's always a bigger fish" there's a feeling that nations inside of the banner at least on paper are obliged to not attack each other in an effort to deter the larger opposition.

I personally though this is a bit of a fantasy, as Greece - Türkiye showcases that situation is far from ideal and wouldn't it make more sense to try to reason and integrate with the larger (outside) nation? As otherwise you're really leaning into a narrative that the "outside" is somehow born evil, which should feel quite hollow to anyone hearing it.