Perhaps it's too difficult to accurately represent. In GT4, the option was available (albeit packaged, compared to your individual component list), yet when it was installed was shown to do absolutely nothing in terms of game play. It was nothing more than a waste of credits, with absolutely no in-game benefit other than simply having it.
I think PD realized this, and has been working on it to un-satisfactory ends. Brake fade is hard to determine, seeing as how everyone brakes differently and different compounds wear differently...
And of coarse, when a system is in place, someone else will be unsatisfied with it, so it'll need a couple of patches. Then even more patches to fix any glitches it causes or that may arise during it's development.
Simply put, would you rather have a brake upgrade option, or more cars and tracks (lets pretend more cars doesn't mean Skylines, Miatas, etc.)?