The reason there are fewer decisions is because buildings are way better understood than software.
I think buildings are better understood in large part because they are not nearly as malleable. Because software is highly malleable by nature, the complexity and scope of decisions can grow much faster than in any other engineering discipline.
I'm convinced malleability of the software isn't the issue; the issue is that the thing that the software is modeling is malleable and generally unknown to the level of detail necessary.
Nobody in a business knows all the business rules, no manager is aware of all the data that their underlings create, manipulate or consume, no individual or single location at FAA really knows all the rules on how traffic control actually works, etc etc etc.
But when we create software to automate any of those things then, at that point, need to have a full understanding of all those things. And then we generally also discover that the rules discovered are inconsistent, violate some other rules or laws, etc. And once those are fixed, then and only then people realize that what they had isn't quite what they wanted. And not only what they want changes, the whole underlying system was evolving at the same time, so it sometimes feel that you're starting again instead of tweaking the implementation (and this may also be why software methodology research focus obsessively on approaches that make things easy to change at a later date)
When someone builds a multistory building, they will build the first floor to support the second floor. And in general it is exceedingly obvious why that is the case.
The problem with software is the abstraction. The first floor and the second floor aren't connected in any physical fashion. It's easy to rip out the first floor after the second is build, and only later realize you cannot support the necessary load.
Buildings have a far smaller state space, and that space is highly decomposable. There are only so many doors in the building that can be open or closed, lights that can be on or off, HVAC zones that can be on or off, or elevators that can be going up or down or stopped at a floor, etc. And few of those states interact.
Software systems have astronomical numbers of states, and while most of the discipline of software engineering is about how to minimize unintended interactions between them while still producing the intended interactions, we still wind up with lots of the unintended kind.
There's a lot more to the state of a building than a human interfaces. When the ambient temperature changes, the state of the building changes. When the wind blows, the state of the building changes. When it rains, the state of the building changes. As materials age, the state of the building changes.
We're just so much more familiar with these forces and states that we can reliably model and design for them, and then (as with your comment) not worry about them anymore.
We take it for granted that our buildings won't fall down in a storm. But the knowledge of how to do that had to be developed and standardized at some point.
For buildings not collapsing, the main thing that matters is that the structure's strength is larger that the applied forces. Edge cases can be solved by adding more material, or more intelligently by having enough redundancy that you have enough strength even if a small number of components fail.
This, of course, does not apply to software functionality - you can't fix bugs by "more CPU power". However, if you look in the places where you can apply this methodology - like cloud services - you find that they are indeed very reliable.
If the materials of software are for-loops or text files, I think one can say that "we" are familiar with them.
A simple program or a simple piece of a large is something one can be very familiar with.
Indeed, the particular parts are more predictable than particular parts of buildings, who behavior changes over time, which literally rather than metaphorically wear-out, which have to simultaneously fulfill a number of functions simultaneously, etc.
So I think it is ultimately a matter of the state-space of the ingredients rather than a lack of familiarity with the materials.
I am not sure malleability is really the core issue... the core issue is that there are many different approaches to making a feature with different trade offs and costs that are not readily apparent at the outset. Not to mention that poor design upfront causes a plethora of issues down the line if you need to scale it.
I think buildings are better understood in large part because they are not nearly as malleable. Because software is highly malleable by nature, the complexity and scope of decisions can grow much faster than in any other engineering discipline.