Why? Cost. Why reinvent something if you don't have to? You can get a full set of STi take-offs for anywhere from $100-$300. You can't even buy koni inserts for that price, and you pretty much have to go new with koni's. Yes you can find a used koni setup, but firstly, it's not going to be nearly as cheap, and secondly they're few and far between. STi suspension comes off all the time, the abundance is there. If you end up damaging the struts or springs, it's quick, cheap, and easy to find a replacement.

As far as the two cars not being compatibler, what exactly is the difference? About 150 pounds or so, big deal. The spring rates are similar enough if you scale them accordingly.

Besides, there's a huge aftermarket for Impreza stuff. If that stuff can be made to fit, then our aftermarket suspension just increased a lot.

It seems like common sense to me that if a replacement part is available that will work okay, then there's no point in trying to design something from scratch. You're more than welcome to though.