It's just your complete lack of awareness. In the Cook County Forest Preserves (Chicagoland) gangs keep fighting pit pulls in outdoor secret camps. The pits get lose and attack walkers/joggers on the MUPs. IIRC there's been one death and several life changed survivors (leg and arm muscles are torn off and the limp is permanently damaged).

My son was mauled by an Akita when he was 7 years old (he's 27 now). So while you can say you've never seen it, you have a first hand account.