The current trend of “soundboxes” lile this > http://cdm.link/2019/05/10cars-electro-acoustic-instruments/ makes me think there’s also a desire to build specific physical tools for specific sounds. And just like most software self-created instruments, those are maybe less focused on tones, scales and score-reading tools, and a little more on very specific physical approaches, a way to reach certain kinds of textures, atmospheres, sensations, more quickly and with a different physical relation to the instrument than when trying to play the right note at the right time with enough dexterity.
So just like these soundboxes do not try to emulate a Clarinet or even a Kalimba or Violin, despite having some tools of the former (bows, strings, plucked metal stripes), I think self created software instruments are not trying to emulate a moog, a prophet 6 or whatever “synth as a classical score-reading, dexterity inducing, piano playing instruments” are out there. It’s about trying to find a different way to connect to sound, prototyping ideas without the overhead of thinking about the perenniality of the instrument, without having to consider how other people might use it and how you should accomodate that.
Just like you absolutely can do what SPCs do on a laptop, you absolutely can do what most diy built soft instruments do with other synths, but still, doesn’t mean you will, and doesn’t mean you’ll enjoy the same things about doing it along the way.
Also a final point, I know a lot more people building their own websites with little knowledge in coding and 0 money, than I know people building their own houses with little knowledge in architecture and 0 money.