Stainless steel brake discs for cars don't exist for a very good reason - cast iron performs better. The coefficient of friction between conventional organic friction materials & typical stainless steels is around 20% lower than it is between the same friction material & cast iron. If stainless steel discs were used the lower coefficient of friction would have to be compensated for by changing something else in the system, e.g. increasing system pressure, bigger brake discs or higher-friction pads.
Cast iron also has cost advantages - it's a cheap material easy to manufacture using very simple technology.
Motorcycles do use stainless steel discs, mainly for cosmetic reasons. They use bigger wheels than cars so have room for bigger discs & high-performance bikes use sintered-metal pads which combine very high friction levels with freedom from fade. Sintered pads could be made for cars; however, apart from the high cost they would be deemed unacceptable by most users because of noise - not such a problem on bikes which don't have a big flimsy metal box to amplify the noise!
Where are we going & why am I in this handcart?