The difficulty of working on lower abstraction layers doesn't scale with the amount of higher layers. Unlike with buildings or bridges, there's no gravity in software, no loads and stresses that need to be collected and routed through foundations and into the ground, or balanced out at the core. In software, you can just redo the foundation, and usually it only affects things immediately connected to it.
A set of analogies for software that are better than civil engineering:
- Assembling puzzles.
- Painting.
- Working on a car that's been cut in half through the middle along its symmetry plane.
- Working on buildings and bridges as a Matrix Lord who lives in the 4th dimension.
All these examples share a crucial characteristic also shared by software: your view into it and construction work is being done in a dimension orthogonal to the dimension along which the artifact does its work. You can see and access (and modify, and replace) any part of it at any time.