As NZeder said it is the price that comes into it as well, I am currently trying to organize a Wilwood group buy with the JSK adaptor kits here on the AusZcar forum and the cost for the front set up alone is around $1000.
The caliper and rotor sets alone are about $550, this is in Aussie dollars of course, whereas I could get a complete set of second hand 4/2 OEM NIssan brake calipers from a z32 for around $300 then just match up some rotors. It is more for an OEM look not a cost thing for myself as I said I am already in the processes of obtaining the Wilwoods.
I actually priced them here through a local supplier and it was more than well over 2K for I think they were Outlaws though but basically a similar product, that was just for the front set not including fitting. I suppose I just couldn't understand why you would not want to use an OEM product that would only enhance the overall appearance of the car.
So far I'm taking it that no one, apart from someone in Japan, does these, LOL NZeder you might have to teach me Japanese dude.