It's because straight off the showroom floor they are running rich and undertuned.  Anyone that has a STi will AT LEAST Stage 1 tune it, which will give it noticeably more power and throttle response.  Plus, if you are just daily driving an STi you get a WRX.  The STi is a pretty crappy ride compared to the WRX, so if you are only going to do the "the occasional blast" you are wasting your money.  And yes, I was referring them being too much to handle at WOT at full performance when you are redlining the engine, doing curves or whatever.  I wasn't referring to it being too much to handle when you are just DD the car.
And to the guy who posted about the transmission, the synchos don't go because the tranny isn't designed the handle the power.  Quite the opposite actually, the synchros go because the tranny isn't designed to be torque shocked from the engine drastically downshifting and switching RPMs.  Hence why you rev match.