ORIGINAL: lov2flyrc
Actually, there are several manufacturers that are using home brewed wheels these days.... Jets-Munt, Jet-Central and even Wren I believe all use their own compressor wheels for some classes of engines.
They only reprofile existing compressor wheels. Like GSR wrote the only manufacturer that mills their own compressors is [link=http://www.hawkturbine.com]Hawk turbine[/link].
As to why not mill out of Magnesium... Well it's weaker and can be a pain to work with. With the compressor being less than 5% of the weight of the engine the savings would be negligible.
Oh, and there's always the risk of a fire after a crash. [8D]