Bro, there is a HUGE market for one off software projects for lots of money.
however I suppose at the point where I need to explain that and all the ways in which the two things are dissimilar it becomes a book in itself.
But at the risk of being uninteresting, I know of incredibly niche software projects that were done for a single rich benefactor as one-offs simply for their personal use-case and no one else. Years in the making and quite well paid. In one case effectively lifetime employment for the sole developer.
They are obviously incredibly rare, but they do exist.
However, they are totally irrelevant to discuss in this context since it's a rounding error of 1% of 1% of current developers who would get such roles either via luck or skill.