That's because here in the US, if someone leaves a package or a backpack sitting, and wanders off, someone just steals it...
Isn't that the truth.
If you filled a cardboard box with stuff that looked like explosives, wrote "BOMB" on the outside and left it on the sidewalk around here, there's a very good chance someone would walk off with it before it was reported to the police (especially if it had visible copper wiring). You couldn't even hide it in a trash can. Someone would dig it out and haul it away in their personal shopping cart.