It's true that, as an atheist, I have no ultimate explanation for existence. But I will sooner expect an answer from cosmology than from the writings of ancient religious writers.
No. The "physicalist" worldview is the only verifiable one. There's no reason to assume the existence of anything else. Since you're the one claiming that there's more than meets the eye, it's your burden to provide proof.
Heck, there's not even an agreed-upon definition of "supernatural"...
The same reasom I care about Islam and conservativism and any other evil under the sun. Christianity is hindering stem cell research, reproductive rights, and the fight for equality. Until Christianity stops budding its nose in our affairs, I'll care a great deal.