[nialo]

The equivalent of source code in civil engineering (and engineering of physical things in general) is the drawings/plans, not the actual bridge. In practice it turns out that drawings for physical objects are often even worse at comments than software, in part because leaving comments on CAD models is so much less convenient than in code.

[YZF]

Not exactly. The drawings I've worked with had lots of annotations on them (e.g. dimensions and manufacturing instructions). I agree it's not a perfect analogy but perhaps uncommented/undocumented source code lies somewhere between the physical thing and the drawings (or CAD file). My point though is that in theory every bit of information can be observed from the physical object it is a poor way of storing that information.