Probably because it costs about half of the blue book value of the car to have one properly installed. Its not just as easy as slapping one on the top of the engine. Most of the time you have to buy a custom fit kit in addition to the supercharger and labor itself. After the install, the engine computer needs to be re-flashed, in order to deal with the new schematics.
Seems the only way this would be logical to me is if you are a gear head, and can do it all in your garage. But if you WERE a gear head, you wouldn't be doing this to a corolla in the first place.
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