Because there is no evidence that we can use as a fundamental bases and be like ''Yeah that's how the world was created''. It's more that there are a series of experiments that we can use. Though we are yet to put them together.
Aight aight, erm... idk if Big bang is 100% definitive cuz physics ain't my subject, but that was thought up because of something to do with Einstein, Red shift and something about radiation all over the universe? I learned the very basic gist of it in philosophy, oddly enough. Something to do with trying to prove that the universe hadn't existed for an infinite amount of time....