Something doesn't seem right about this algorithm you used to draw the rectangles. If you used a similar algorithm to define circles, I feel like you would get a artificially high amount of circularness for countries that are not that circular.
Or probably just any kind of polygon from triangle to whatever you choose to find. The common property is probably rather convex shape than rectangularness or anything arbitrary like that. Maybe because convex borders are easier to protect or less threatening to neighbors or something like that?