You've got it WAY backwards. Guitar isn't just tempo and velocities like piano is. If you don't understand that, well, there's no point in trying to explain. Guitar is not "easy" to fake. It's the HARDEST instrument to fake (and has been forever until something like Electri6ity came out). Humanizing piano is probably the easiest thing to do, it's only the actual coming up with the improv that's hard. Getting it to sound realistic is nothing but understanding how to adjust velocities (and edit tempo, but that's more from your head and less a realism thing). My example is Willrock's remix. There is a small synthetic quality to it, but it's more than enough to fool the common man (and will get past the judges, if the OP tries to sub).
And yes, guitars play one note at a time for leads... and 3 for powerchords, actually. But as for piano, there are zero legato sounds required. It's just pressing different keys. Guitars need to include fretboard slides and legato picking like hammer ons and pull offs, and switching strings (accounting for muting the strings properly or being able to know what's still ringing when you shift to a different string) It's ridiculously complicated to sequence a guitar able to fool someone other than the common man. It's even harder on a piano roll because you don't even know what string you're on, you only know what note you're playing.
And guitars are the most dynamic through an amp. Vibrato-type subtleties are things you can actually pick up through an amp, and stuff you do on an amped electric that you can't hear on a regular acoustic. Without a well scripted and expensive sample library, humanizing expressive guitar playing is IMPOSSIBLE. And I'm talking about lead playing, not rhythm guitar playing. But I'm derailing this thread.
The point is, to the OP, that humanizing piano isn't hard. The hard part is coming up with expressive improv. It's hard to do from your head to your speakers, because a lot of it is impulse.