Answer: it was designed to be that way by God who made earth habitable for his creation. Sure that statement takes faith to believe, but probably less faith than any crazy idea someone else can come up with.

I think it's worth noting that a model was created by the author to try to figure out the probability of the Earth maintaining habitability over time. He had to design planets in his model to include various feedback mechanisms to maintain habitability. It seems even less likely that those mechanisms would spontaneously arise on a given planet without the intervention/direction of an intelligent being.