Consider the solitary attribute of stone.
Unlike other more fluid, cohesive elements like fire water and air, the basic quality of stones is their detachment.
Stones keep their individual shapes and sizes ferociously. They keep their distance too, even when they are collectively compressed together tightly by their own great weight.
It is this dispassionate 'disconnectivity' which makes them very useful for building with. The very thing that keeps a dry stone bridge or arch together is the the solitary resistance of stones to becoming 'attached'.
Stones don't merge . While a slurry of Portland cement, aggregate and water will eventually harden to become a single stiff lump of concrete matter, stones properly fitted together dry create a structural network that not only has dynamic strength but the capacity to yield.
Bridges of steel and wood are physically fastened together. The various materials 'tug' on each other and as long as they continue to stay 'bonded' the structure stays together .
By contrast, it could be argued that the stones in an dry laid arch stay up by repelling each other.
