I think because it's the simplest way to represent conflict. There are many types of conflict, but punching someone in the face is an obvious one.

Economic, emotional, verbal conflict, for example, are hard to represent. The outcome may not be immediate, and the result may not be obvious.

Now combat is immediate. I shoot. If it bleeds, that means I hit it. If it falls down, it is dead. If it's not dead, shoot it again.