Why don't companies make turbos with carbon fiber parts?

There are other forms of carbon besides carbon fiber reinforced epoxy. There are currently pistons and disc brake pads made from C-C (carbon-carbon) and C-SiC (carbon-silicon carbide). These will handle the loads and heat. I'm not sure what is holding back changing the exhaust turbine in turbos to C-C designs. It may be the more complicated geometry required of the turbine impeller.