Music publishers did, over the course of the last 400 years. It's whatever, you are absolutely right. Ever since the advent of modernism most traditional conventions are open to being retooled, appropriated, and even rejected. I do believe cataloging your work is important, I do as well. However I don't refer to it as opus numbers, because it does seem misleading. It is important to note that traditionally a single opus number could denote a collection of works, not each individually. If a set of pieces was published together it may have a single opus number for the whole set, and then each piece would have a subnumber associated with it. This is generally why a lot of prolific composers didn't have really high opus numbers comparable to their output; most works wouldn't be published at all, especially shorter ones. When shorter ones were published, it tended to be in collections that would all carry a single opus number.
My cataloging doesn't fit anything resembling that, so instead of labeling something as Opus # such and such, I simply place the number after the title. For example a title might look something like "Beautiful Love Song (#235)"
When I was a kid, I would assign a catalogue number to each individual movement, and then another one to the piece as a whole. This is probably not how I would do it today, but I don't want to break my own conventions at this point, and I don't want to renumber the whole catalogue.