Economics. It's way cheaper to buy a few old instruments (buy extra in case you want to do multiple takes) and just record them being crushed than to pay a team computer artists for weeks to simulate the physics and draw this all in photorealistic CG.
Why do you think its insane to model realistic looking explosions? It's done all the time. Even if it started as a practical prop it was certainly doctored to all hell. Stone statues don't squish and guitars don't actually explode...
If you look through it you can see the top of the guitar is even cut off at the neck, either as a prop or digitally.
