Actually overenginnering is the thing with pre-modern structures, like the bottom bridge here. Survivorship bias played a role, where things they don't build to last, evidently don't last to this day, but mostly it's because they don't really understand the math behind all of it so they take the most conservative and the tried and tested rules of thumb when doing big structures. This is why big projects back then can take decades to complete.

In the modern day, we design specifically to balance durability and cost, and we are confident of our maths and understanding of material science to use the least amount that does the work for the design life that we choose.