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.
I know many people like to recite the mantra that “abiogenesis is not evolution,” but it’s a cop-out. Evolution is about a plurality of natural mechanisms that generate diversity. It includes molecular biases towards certain solutions and chance events that set up potential change as well as selection that refines existing variation. Abiogenesis research proposes similar principles that led to early chemical evolution. Tossing that work into a special-case ghetto that exempts you from explaining it is cheating, and ignores the fact that life is chemistry.
-PZ Myers