Infrastructure is not permanent. 'We' don't build roads and bridges to last, governments might run a cost-benefit analysis or a net present value and build a project out to where it makes sense given time money and interest. Bridges need repair, asphalt cracks and wears, re-bar rusts and compromises concrete from the inside, infrastructure depreciates. The ancient world, some of those things, are built to last--- because they're made from solid stone and such.