Why do fantasy worlds use "christian crosses"?

Holy crap! Holy crap! It seems like every fantasy story be it games, books, ending with the letter "S" that has a cross in it, it's a christian cross. Why? Why do fantasy worlds have a cross that looks just like the cross that "Jesus" was crucified on? Why can't any other historically real cross be used in a "fantasy" setting? I just don't understand why, other than wanting to tell a story where christians are the "good" guys (looking at you Inheritance Cycle).<br><br> developmental edit to add: This is not meant as an attack on any religion. The question is about why a symbol of a historical real world religion is used in a fantasy setting.<br>Second edit: I am not picking on christianity over any other religion. I am attacking the fact that a religions symbol is used in fictional settings that have no connection to that religion.