Why Can't I Get A Pentakill???

I really want a penta, so i play quite a bit of Katarina. But every time i get fed, one of two things happen. 1) someone steals my penta or 2) the enemy team surrenders before we can even take inhibitor. Now I'm getting a little confused. Can anyone give me a reason or advice on how to get a penta?? I swear, i dont even remember the last time I won a game where the other team doesnt surrender.