Sure - like "why not just by the AMG model?"

For the same three reasons: 1) It may be hard to locate the AMG (or in your case the SRT-6) 2) the insurance on the SRT-6 is likely much more expensive than the baseline Crossfire, and 3) some guys just like the joy of doing it - of being able to say "I supercharged it myself". They enjoy playing with their car. Its part of the fun.

Does a supercharger kit make sense for everyone? Certainly not. But across the SEMA industry, they sell and they do sell well to those of the right mindset.