Why is he so incredible?
Because he had personality, his own rules, his own voice, his own tone, his own style, his own sound, as well as he had something to say. Not many modern so called pianists can say the same. Whether you like his personality, voice, tone, interpretations, etc., or not is completely different thing, but no one can deny the fact that one can recognize Horowitz from only a couple notes, or from one single chord he plays. For me this is probably one of the most important qualities in an artist--to be unique. There were many great pianists in the music history, however, not many had this unique quality (to be recognised just from a few notes)--Rachmaninov, Hoffmann, Gilels, Gould, Horowitz...
I might even agree there are pianists who maybe play piano better than Horowitz (don't confuse with "to be better"

), however, the difference is, if Horowitz played like crap (in the later years, under medication), still in every note one could recognize a great artist. Many others could play very well, or even great, but still sound like... crap artists.
Best, M