IMO, it's very smart decision to not plate shanks.
For first, it's cost effective for manufacturing, as Doug said.
For second, it's cost effective for customer, who can buy 2 different shanks in raw brass instead of one silver-plated.
And, thirdly, WHAT part of mouthpiece usually shows its "age" or signs of usage?
Yes, correct answer is SHANK!
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So, why to plate a part which anyway will have wear?