At range?
Because you don't miss with a knife. You may not inflict the wound you wanted to, but you don't miss. Knives don't kick. Knives don't produce a sound that people have learned to associate with extreme violence. You don't have to (but obviously can) train a man in the proper operation of a knife. A knife is only an extension of the hand. A gun is an extension of the eye.
"Easier" seems a bit too broad, is all. A gun is obviously a more powerful tool.
I do agree with the basic point. It just depends on what the meaning of "is" is, I guess. Easier is not the word I would have chosen.