Custom software, or software for a small audience obviously is scarce, but a word processor or operating system with similar complexity that can be used by millions, if not billions of users, should not cost more in total.
Open source (free) software would not even be possible if it were as scarce as, say, oil or apples.
In what world is software comparable to apples or oil? We need a different analogy.
Note that I am referring to commodity software (because that is what this thread is about). The reproduction costs are negligible compared to the software development cost.
Perhaps for a better understanding of my point, consider book publishing, or limited edition prints of art. The scarcity is almost completely artificial.
Microsoft and Oracle, for example, have grown to be extremely large companies due to this possibility of selling the same thing at nearly no extra cost.
If you think this is normal, then that's fine, but commodity software is most certainly (as you hint at yourself) not a good fit for economic models based on scarcity.
An extreme example to make a point.
A less scarce, but still scarce software product is one that sells for $100 or $10 or even $1.
Software can be free (as in free beer) and not scarce. But many software products wouldn’t have existed without customers who pay for it, with the demand level and price setting the scarcity.