I was discussing exactly this with my dad (a retired civil engineer and expert on bridges) just the other day. I (like you) thought that that was the way bridges were built, but it turns out it civil engineering is much more like software development.
Bridges are built with detailed but slightly vague plans: there are always many unknowns that can only be fully discovered during the building process mostly to do with the environment in which the bridge is built and the exact behaviour of the often custom-designed and custom-built components being used.
The engineers are required to re-design parts and to rework components on-the-hoof. Sometimes new prototypes are built and tested to destruction. Some components go through stages of iteration.
It is never certain that a major construction project will come in on-budget and on-time.
Bridges are built with detailed but slightly vague plans: there are always many unknowns that can only be fully discovered during the building process mostly to do with the environment in which the bridge is built and the exact behaviour of the often custom-designed and custom-built components being used.
The engineers are required to re-design parts and to rework components on-the-hoof. Sometimes new prototypes are built and tested to destruction. Some components go through stages of iteration.
It is never certain that a major construction project will come in on-budget and on-time.