Because when you see the “shotgun trap”, you’re not really seeing the shotgun - you’re seeing the trigger of the trap. Imagine the shotgun is buried somewhere nearby, and the trigger is just a piece of rope under tension that fires when it breaks.
Now, how do you disarm this? How do you find the shotgun? Obviously, you could dig around for it… but that’s exactly the sort of thing that risks setting off the trap in a way that you get shot!
There has been discussion of ACTUALLY moving the shotgun to a different square, and having a generic “you see some sort of trap trigger here” sort of trap, (and being able to tie it to things like doors, so you can have the shotgun across the room from the door or around a corner where it’s hard to reach and disarm directly) but I’m not sure what the ultimate result will be. For now, though, assume the shotgun is inaccessible or it’s location is unknown until you disarm the trigger, and that’s what you’re actually trying to do here.