You're confused about fundamental vs. derived theories in science. As another poster said, when you start asking questions like why do natural laws exist?, "this is not the realm of science": it's philosophy.
We can explain evolution in terms of more fundamental mechanisms: selection, mutation, etc. With gravity, there is (currently) no underlying theory. And if we found one, you'd just ask why that theory exists. At some point, you hit the bottom, and it doesn't mean you understand the theory "less", it just means you've finally reached something that isn't describable in terms of something else.
Watch Feynman's attempt to explain this in the context of magnetism to understand this better.