Maintaining things is not necessarily less costly or even viable, compared to building things.

Also, labor costs have gone up dramatically due to a variety of factors. Increased safety standards, less supply of labor relative to demand, increased liabilities due to tort laws, etc.

Also, borrowing from the future to build today is easy when population is exploding and you know everyone is having 3+ kids which will expand the tax base. That has not been true for quite a few decades now, with tax bases declining in many areas and political appetite for subsidizing those areas is waning, in my opinion.

Less future growth means more restrained borrowing today. There is also a significant and increasing amount of the country’s resources being spent on keeping old people alive that was not the case when all the old infrastructure was built.

Have there been no improvements to bridge building technology, that would make it cheaper today?

I assume there have been, but based on all the reported costs of building infrastructure, I concluded that any reductions in costs must have been greatly offset by increases in costs.