2 reasons,
one is CAMS does not allow engine changes and can only run turbo if the car came from the factory with a turbo
the other is cost, you can build a good motor for less than the cost of a millington and then try and find someone who knows enough about it to tune and maintain it.
one wonders why rallying is dying in NSW under CAMS when we have other options that do allow engine changes. Cue AMSAG/ASAA debate