I used to design and inspect bridges before coming to evil petro chem. Notice the corrosion on the galvanizing at failure points. Image: https://pbs.twimg.com/media/BK_6txyCEAATwtL.jpg
Bridges require constant maintenance and up keep that costs money. In extreme conditions (north gets cold + corrosive ice & snow abatement... coasts get salt coated), the bridges are not inspected frequent enough in my opinion. Most states/municipalities have a rotation of bridges they check. It is literally a full time job for a crew of people that some bridges go 10+ years between inspections.
Most of our failed inspections I came across were due to unreported roadway incidents, water traffic collisions, and pigeon poop.
A huge problem we are facing is that the interstate system was funded by war money. The system was mostly designed for 30-40 year life span (you do the math). Now there is no more war money to keep up with maintenance, let alone growth and widening. The money has to come from somewhere and it will be interesting on where it comes from.