I don't think this is just a matter of bureaucratic inertia. Both the personal and societal risk is much greater when a bridge collapses (e.g., Minneapolis) then when a bridge is over-engineered. An engineer is much less likely to have sleepless nights, be demoted, or lose his or her job over an over-engineered bridge. We tend to remember engineering failures - Tacoma Narrows, San Fransiquito - much more than over-built wastes of resources.