I'd have to say part of it lies in how we define things. The Big Bang is both deduced and defined to be the first moment. The math (or rather, the simplifications that I can understand) is about conditionality: you need a Big Bang to create timespace, and we can only understand it from within our native timespace. There can be no "before" or "outside" without introducing fundamentally incomprehensible space/time/?? continua of "other" dimensionality.
The difference between Big Bang theory and any other mythos of creation, however, is in the fine print: "We're open to being proven wrong about this." I respect that. cn