Bridge building is only minimally adversarial. You have to make the bridge strong against chance, natural threats, and casual vandals. You don't have to make

Bridge building is only minimally adversarial. You have to make the bridge strong against chance, natural threats, and casual vandals. You don't have to make the bridge more than minimally strong against attack by a foreign military-- or even some wacko with a van full of ammonium nitrate, and you probably can't (or at least it would be utterly budget breaking to make a serious attempt and doing so). While if you don't make your computer strong against fairly powerful attackers it will get attacked, because the economics of powerful computer attacks make them much less costly to create, almost free to deploy, and relatively low risk of consequences for using them. Fortunately, defending computers from attack also has good economics compared to defending bridges.