Keyboards much easier
Unfortuantely it's always been a lot easier to do things like that with a keyboard than a guitar.
The keyboard is digital and the notes are switches (not withstanding aftertouch et al but that's still just digital info as far as MIDI is concerned) so it's easy to caputure that information and use it to trigger actions.
Guitars however are a whole different boiling vessel of aquatic creatures. They way they sound and play are affected by a great many factors - not just what note you're playing but whether you're playing chords; using harmonics; tapping; sliding; pouring lighter fuel and setting fire to... etc
With a MIDI hexaphonic pickup you can at least pick up which string is being played and isolate the tone on that string (to an extent, although modern ones are getting very good). I'd venture this is how the "Rock Band Pro Guitar" works.
Without a MIDI pickup you're relying purely on the line signal from the jack which is purely analogue. If you're playing a single note it can work out what that note is (that would be how tuners work) - say a C - but it can't tell you WHICH C it's playing as with the guitar the same note (in the same octave) appears in several places on the fretboard and you won't know from the line output which one is being held down.
That's before you get into chord voicings, fingerings, harmonics and alternate tunings.
I'd be very interested to see how well this technology works (and how much it actually attempts to do - i.e. is it just going to tell you if you're playing the right tonality or somehow try to work out if you're correctly playing the right fret/chord fingering) though. Could, if it's good, lead to some really nice inovations in the "real" guitar world... or it could just be a gimic.
We'll see :)
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... and jack sockets on guitars aren't like SCART. They are all compatible by nature :)