… the naturalist believes that beneath every natural phenomenon there exists yet another natural phenomenon. If explanation by reference to an endless stack of large turtles is silly, then an explanation by reference to an endless stack of natural phenomena would be equally so. The naturalist's answer for the origin of life, therefore, is some natural phenomenon. (Which one is not particularly relevant.) When you ask them how that natural phenomenon came to be, their response boils down to: "It's natural phenomena all the way down!"
-Pete Chadwell
Natural explanations are assumed because there is no evidence for the supernatural. Science doesn’t claim to know the origin of life; this is a claim only creationists make.