why the metal plate of a capacitor is defined to be made of air instead of being a kind of metal in "capacitance model 12689"

Hi all,

I was looking at the "computing capacitance" model 12689, and I figued out that the metal plate material is defined to be made of air.
Why we do not define the metal plate material as a kind of metal, and leave it to be built by air?!
The link to access the model is:

http://www.comsol.com/showroom/documentation/model/12689/