Because, like it or not, that's how we build big things. A bridge connects two otherwise separate geographical regions. To a government it's an abstract piece

Because, like it or not, that's how we build big things. A bridge connects two otherwise separate geographical regions. To a government it's an abstract piece of infrastructure that will have economic and social impacts. To users it's a convenience that will change the way they plan journeys. To traffic planners it's another edge in a graph. To cartographers it's another line on a map. To road builders it's another surface to tarmac. To geologists it sits on a (hopefully) stable foundation that isn't expected to move or subside for at least a few hundred years. To cement people it's made of a particular blend that's the product of a specialised industry and expected to last for a hundred years. To metal workers it's reinforced with steel with particular strengths and weaknesses. Nobody understands it all. Abstraction is not the source of complexity, abstraction is how we deal with complexity. The complexity is just there whether you want it or not. You think it's easy because you're the guy walking across the bridge.