One of the little secrets of this “premium” material is that while it’s certainly a more expensive raw material than plastic, it’s one of few appropriate materials, and almost the only metal, that’s even remotely cost-effective to machine from solid stock (versus laser or plasma cutting of sheet-stock). For manufacturing at the scale of most custom keyboards, it’s kinda the only truly viable option. Machinery and especially tooling to injection mold or even vacuum form a plastic keyboard case at the detail a custom Alu case needs would be like basically buying a new milling machine for every design. Then, given that machining is a subtractive technique, there’s little benefit beyond shipping weight to make a specific design lighter, so at all price points except the very bottom where per-unit costs finally justify plastic, we get lots and lots of aluminium rectangles.
This is not to say the aluminum rectangles cannot be made with care or be beautiful, or even that they’re overpriced as a general category, just that it’s not purely an aesthetic or premium choice to do CNC aluminum. It’s more of a happy accident that what it’s good at dovetails with what hobbyists like.