Because it like every other notion of man is sin laden and deceitful. It is warped and not clear eyed. It is tainted with man's noxious mind. It is debilitated and unable to apprehend the truth of God. It can grasp portions, but not the essence.
Natural faith trusts in that which it can apprehend.
The natural faith of man believes all that its corrupt heart can conjure and fashion. It is not a matter of sincerity or amount of faith. It is a matter of origin. As long as faith originates with man it will be sin laden and veined with error. It will not apprehend the truth.