"It’s a flaw in our argument, for sure. By any reading of evolutionary theory, creationists ought to have died out ages ago. They serve no function in the planet’s ecosystem, and no other species has survived so long while in such fundamental disagreement with observable reality. If I wasn’t such an ardent believer in secular materialism, I’d wager this is really troubling Darwin in the afterlife." ~ Richard Dawkins
Apparently there hasn't been enough evolutionary pressure to weed them out yet (that whole faith-based medicine needs to catch on just a bit better).
Worse still, it seems the believers are apt to have the most children so (even if a few die due to inadequate medical attention) they are probably breeding faster than atheists.
So maybe they even have an evolutionary edge due to their willingness to have more children based on their beliefs, even if evidence points to it being a better idea (for you, your kids, society) to reasonably limit your number of offspring.
It would seem our only hope is educating their spawn.