Because there's not a shred of evidence in support of such a contention. It's that simple.
Again why is lack of hard evidence, evidence itself that something doesent exist? History has again and again proven that lack of evidence there and then is not proof that something does not exist. 2 years ago we got finally proof that a pocket inside the ice during iceage was warm, with forest and animals. I can probably make a list of hundred other examples where lack of evidence has been just that, lack of evidence, not proof for something else.