Here's a question: Where THE FUCK did they get all that platinum? I mean you can say "It's not our world, platinum could be common" 'till you're blue in the face but that seems just a little ridiculous.
I'm not super annoyed by it, but it's just a little weird they picked a metal that's not only rare in real life but costs over $20,000 a pound. They could have easily made up a metal, or used silver or something.
Any metal can be purified surely. But some metals have some really really bad problems when pure. At least platinum is incredibly unreactive if it likely doesn't have the tensile strength to be useful in that application.
In reply to TBSWK, #8850: Picking a metal that is rare is a very good reason, imo: It's one of the rarest elements found in earth's crust, sometimes found in the ores of other metals in small amounts. Additionally, Platinum is the least reactive metal (non-reactive).
These reasons could be taken to mean that Earthbenders are not able to bend it because they are less familiar with it and it is very refined earth that retains its purity, as it will not corrode.
You could also say that platinum just doesn't know how to react to an earthbender advances. Would explain why the Hiroshi & the equalists have taken such a shining to it. =D I'll show myself out...