I think a better question is, "what's for dinner?" Yeah, I also wonder how he got from The Birth of the Cool to Bitches Brew, but he still managed to do it.
One thing that all jazz has in common is that it is improvisational based. So if Kenny G's music is entirely composed before it's played, then it's not jazz, it's just instrumental music that's made to sound like jazz.
The thing I like about jazz is not just the emotional impact -- although that is essential -- is the intellectual stimulation as well. It's really the best of both worlds. On the best jazz I've heard, you can almost get inside the person's head and 'hear' him composing on the fly. The ones that I admire most are the ones that are 'composing' faster than anything I can imagine (that doesn't necessarily mean playing faster, but they're anticipating what to play, and picking the perfect note or set of notes [or combination of notes and silences, to the pedantic] better than I can at that speed).