In many states, there are simply too many bridges that are coming due at the same time. When you take into account budgeting, state and local policies on detours (for safety reasons ie emergency services being able to get across town in 5 minutes max), and record flooding taking out entire bridges you start to see the issue.

We recently had a situation where one major thoroughfare was closed for planned reconstruction, and 2 other bridges were closed after a hurrican came through and caused irreperal damage to them. This brought traffic and commuting to grid lock in the area, but the replacement bridges could not be engineered fast enough and the other bridge was literally demolished down to its foundations.

Not every bridge has to be substantial either. In a lot of states in the North East there are thousands of tiny bridges over creeks. And many of them are well over 100 years old but just so inconvenient to replace.