When ever you ask a question beginning with the words "why don't they..." the answer is always money. That's the answer here as well; the cost of stainless in a disposable part is prohibitive. Now... one wonders if somebody might one day make on that was intended to be rebuilt, and charge accordingly. This seems unlikely however. For one thing, they make a lot of money selling replacements. For another, the design of a standard coil head is stupid if it's intended to be rebuilt. It makes much better sense to design a different part like the Subtank RBA deck for that purpose. I don't know, not having one, but I'd be surprised if the RBA deck isn't stainless.