The infrastructure is PURPOSELY designed to have a limited lifetime. It is called "depreciation", whether by physical deterioration, or for functional reasons, or simply because it costs too much to make the necessary improvements so its use conforms with current demands. There is also the continuous re-employment aspect, as in the case of roadways and urban development. Some re-use may be gained by changing things from their original function to some new function demended by economics ot technological change, but the fact is, the world is continually being reinvented or rebuilt, and sometimes, some aspects just fall through the cracks until the situation gets noticed.
Occasionally (well, a LOT of the time), a piece of work is done without considering the future when it must be either dismantled, or additional improvements need be made to restore it to some degree of usefulness. Then, replacing or reconstruction gets much more expensive. Or the design may have been a million-dollar idea, and there was a fifty-cent execution. Which only increases the overall cost, as the original structure put in place must first be ripped out, and a real job of finishing things right must be done.
or it might just be that politicians are reactionaries...
nothing gets done until something tragic occurs...