I have a real hard time with all three of the primary answers...
Cost: as already pointed out, some of the higher end devices are already priced orders of magnitude over the cost of sufficient copper. The base material cost is irrelevant when the market will absorb that base cost and fail to notice the increase in price.
Durability vs machinability: copper is a little gummy to cut, but it's really not -that- difficult to cut, thread, etc. it's plent durable for a mod, and the area for threads would benefit from work hardening, which would make threading much easier by making the metal less sticky in the cut, while simultaneously making the resulting threads much more durable.
Corrosion: as if anyone spends two bills or more on a high end mechanical, but then can't be bothered to wipe it down now and again, rub it with some wadding polish twice a year, and generally just take care of it like any other $200 luxury item they might buy.
All of these problems are solve able, or weren't really 'problems' to begin with. I'm going to have to look into that list above and see if any of the options appeal to me as far as build and layout.