I'm less of an audio artist than a visual artist (which I do professionally to some extent) but I imagine the reasons are similar to why few people make visual art using scripts and imagemagick or some similar workflow.
Most creative output is birthed in a more freeform state of play, to some extent, rather than being reasoned about and assembled like code. Coding itself can be playful, but when your goal is expressing emotions and ideas in art, having to pipe that through a rigid, logical process is much less expressive than grabbing something, even a virtual something, and making some sort of gesture with it. So unless you're trying to express something algorithmic, code is a layer of abstraction that operates in a very different way with persnickety bounds that just aren't useful in most creative expression.
I've done generative design using action script in Photoshop and made some really cool algorithmic photo collages, but even as a long-time developer, the frequency with which I think "this could look cool if I set up a function to do it like this" is pretty rare.