Because you can get the same action/effect by either grabbing the collar of the shoulder pads OR grabbing the collar of the jersey.
Take a look at these two videos
Grabbing the collar of the shoulder pads:
Shoulder pads
Grabbing the collar of the jersey only:
Jersey only
Note: one of these is college, and one is NFL, but the basics of the rule are the same for FED, NCAA, and NFL.
These are both fouls... if "grabbing the inside of the jersey" wasn't mentioned in the rule, the second one wouldn't be a foul.