There are quite a few responses, but I still want to point out a main difference more clearly:
There are natural-intelligence (human) agents translating the diagram to "code" (bricks).
There is a lot of problem fixing going on done by the construction crews, cursing at the architects (sometimes, or just going with the flow and what comes with the job).
That is the same with software:
If you give good developers diagrams those human agents too will be able to produce useful software from it, no matter the flaws in the diagrams, as long as they understand the intent and are motivated to solve the problems.
There are natural-intelligence (human) agents translating the diagram to "code" (bricks).
There is a lot of problem fixing going on done by the construction crews, cursing at the architects (sometimes, or just going with the flow and what comes with the job).
That is the same with software:
If you give good developers diagrams those human agents too will be able to produce useful software from it, no matter the flaws in the diagrams, as long as they understand the intent and are motivated to solve the problems.