A too-easy question
Ooh! Ooh! I know! I know!
I think I know the answer to this one:
Because it's old.
Poor creationists. They have to labor so diligently, so faithfully, and so blindly, to rationalize their God's duplicity. He may have implied in his holy book that the universe was created a scant few thousands of years ago, but then he went and made it look billions of years old. Either their God is a liar or he is a prankster. Loki or Coyote. It's not a very satisfying choice, is it?