Constructor Theory: All things not explicitly forbidden by the laws of physics can exist. The answer to "Why does life exist?" is: "because the laws of physics do not explicitly prevent it."
Seems like an interesting take. I'm not convinced that it's at all useful, but it's not obviously flawed or vacuous.
Sidenote: I've never felt that the existence of life was as mystifying a physical problem as the existence of black holes, or expanding space time, or stars. Why people are mystified by life but blithely accept the existence of distant fusion reactors the size of our solar system is beyond me.