Well that's partly coz it's real and not a digitised sample fed through an algorithm on a small(ish) backplane bus. The processing required and backplane width (>128bit (not 32 or 64bit)) to lower/raise the pitch in real time and still sound reasonable is not trivial, if you combine it with the direct signal it's kind of usable, sounds like an effect (which in some ways it is), but if you "replace" the signal with the changed pitch, that's a whole new story. There's this whole misnomer thinking once it's digital you can do what you like to it and all will be fine
dsp rules (not), it's crap, yes you can do lots of things (with "varying" quality), and it has its place, but apart from all that, you have a frequency/vibration under your finger which is now different to the output.... not good IMHO.
Not trying to poo poo the idea, and great tool for working out songs etc and special FXs but if you need guitars in different tunings, then set them up like that and use them. I understand the convenience factor, but you'd need to spend allot of $s to get something to digitise the signal and change pitch and still sound good/real.