Scarcity in physical objects is rather different than spectrum. For one, market demand encourages things like recycling, more exploration, etc. for platinum. Spectrum scarcity encourages more efficient use, but until we get our promised physics-violation-devices, can't make/find more.

With spectrum, there's also a real-time use component. People who consume platinum generally don't consume it in a way that in any way resembles communication use. This is related to another key reason governments hold on to the control of spectrum allocation: military interests.