By the 1940s, a stone bridge that might take 100 man-years of labor to create could be replaced by a bailey bridge that goes into place overnight. I'm sure if we wanted to create a bridge that lasts 300 years, we could dedicate more resources to it, but it would be grossly wasteful to do so - you don't know how long the current requirements are going to even be relevant. It's better to build a bridge that lasts for 50-60 years, and have the option value of replacing it with a better structure down the road.
And then we get to where we are now a lot faster. . .
"Over 90,000 miles of roads and 71,000 bridges are in dangerous disrepair, according to a 2010 U.S. PIRG study."