It's because it's a playalong meant for you to play the other guitar part. For the full peformance with both guitars sounding you will need to have the "Peformance" of the song instead of the "Jam along."
The peformance video is exactly the same as the jam along video but it has all the guitar parts with the sound.
The jam along will allow you to see what the instructor is doing but leaves it up to you to play that guitar part.
Hope this helps.
JJ