There is simply no reason otherwise. Did you know that when G meets an I, it is a grammatically correct choice to make it soft? Like giant, gist, ginger, and giraffe? So, incidentally, Gif can legitimately be pronounced Jif and Gif. Its just how the word is created that decides which.
But, the creator of the format says it is Jif. And just like giraffe, we must follow that choice because he created the word. We do not pronounce giraffe with a hard G because that is not how the word was created. You must be able to see it. You must know it by now. You can't win. It's pointless to keep fighting. Why, Why? Why do you people persist?