I find it difficult to believe that breaking down steel to be 3d printed into large structures for a bridge is faster or more energy efficient than casting the parts instead.

We can indeed print steel with direct metal laser sintering. I think that the object needs heat treatment afterwards, though to be fair it is almost ten years since I properly read up on it and things have probably advanced since then

Maybe, we could just print off rectangular prism-shaped modules, around the right size to fit in a hand, and then assemble them on site. We could even make them out of ordinary clay and fire them for strength. I wonder why nobody has thought of that. /s

3D printing has it's place, but more conventional methods have theirs too. If you are counting on a lot of human labour anyway you might as well not reinvent the wheel.