Why do Christians oppose magic as made up and evil?

Christians all over have zealously argued that magic is evil and not real, hence why they boycott things like dungeons and dragons and Harry Potter. My real qualm here is that if magic is pagan and evil then why is the bible full of it? or are God and Jesus the only ones allowed to wield the great and powerful magic. I mean giants, floods that cover the earth, curing diseases, transmutation, rains of fire, angels and devils, are all in the Bible… why not throw in some elves and wand waving?