Spoiler: they don't, yet they do, Theseus' ship-style. For all the centuries-old bridges still in use around here, there's only a few bits and pieces left from the original, the rest has been replaced multiple times over as a part of maintenance.
The difference here is probably maintainability: for a stone arch design, you can prop it up for safety, replace a few stones, and voila, like new. For a cable-stayed and/or concrete design...not really. In other words, the bridges that have "lasted for centuries" have hot-swappable parts ;)
The difference here is probably maintainability: for a stone arch design, you can prop it up for safety, replace a few stones, and voila, like new. For a cable-stayed and/or concrete design...not really. In other words, the bridges that have "lasted for centuries" have hot-swappable parts ;)