I think the most important differentiator is that the function of a bridge is extremely simple, a good design has very clear success criteria, and the materials

I think the most important differentiator is that the function of a bridge is extremely simple, a good design has very clear success criteria, and the materials and physics underlying them do not change. That's not to say it's easy, but rather that we've had thousands of years of practice and refinement without moving the goal posts. Contrast with software: Many large software projects fail to even achieve a coherent and feasible high-level spec of what the system needs to do, let alone how all the details will work. Once you get past that phase, you find that the actual design and build of software systems relies on so many layers of abstraction that practitioners have to make a lot of assumptions in order to get anything done. In practice it works because we tend to keep the lower levels stable and coalesce around a relatively small number of proven stacks, but unlike the hard engineers knowledge of materials and physics, the actual platforms are all human defined and subject to change. Imagine how civic engineering would work if physics were just code written by humans that never stopped monkeying with it.