seems like an easy result, but you have to take into account the weight of the structure as built along with the traffic upon it.
I'm sure the romans never calculated 50 loaded semi trailers with thousands of pounds inside parked in a static position on top...
nor did they foretell flooding of such an immense nature over the rivers they were traversing.
If you build it too stiff, one small failure could cause a 'domino' effect that could make the whole structure collapse.
The redundancy that's built in now is what makes them last. Of course you can point to structures like the Verrazano but for the most part, engineers and architects do get it right.
Natural disaster is not a card anyone throws down in their bids, even though we might hint at it.
Even when framing a new skyscraper we use 'floating' studs and extra deep track to be assured that as the building settles, the walls will too. Even the plumbing and HVAC is slip jointed for the same purpose.
As far as bridges go, the concrete deck is really the only drawback. It takes the stress and shows the wear because it's the only part of the bridge that cannot move without breaking.
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