They left it out for the same reason that Newton left out the origin of matter in his descriptions of the motions of matter. It's outside the scope of the theory
But still it leaves out the penultimate question(of who or what created life), and I beleive that is the creationists point, kinda of like the adage "can't see the forest for the trees".
FWIW, I side on the creationist side, they are least being honest in including the question of who or what created life.