The Great Assumption

Because?

Because we have no evidence of what came before the event colloquially known as the big bang or even if before has any meaning in the context. Our best current cosmological model does not make any claims concerning a "beginning".

Is absence of evidence evidence of absence? Say if no evidence was found at the scene of a murder, would you then dismiss it saying, "Well there is no evidence so nobody did it."  Also how can you be so sure there is no evidence? Perhaps you are not looking in the right place.

If there were truly no evidence that would mean no body... no blood... no signs of a struggle. I would gave to conclude that no evidence of murder = no murder and no murder = no murderer. At most we have a missing person case.

Perhaps I should clarify that there is no sufficient evidence of any event before by which I mean observable demonstrable evidence (preferably subjected to the peer review process). If you have any such evidence you may be entitled to a Nobel prize. 

Our current understanding of physics breaks down once you get smaller than a planck length. So it is currently beyond the scope of modern science to say where the initial energy came from. We can only peek at what happened in the first initial moments of cosmic expansion.

Something is unfalsifiable whenever:

X is claimed to be true, whenever X cannot possible be proven false.

From that understanding, the "why" of the universe could be claimed to be any arbitrary standard, and asserted true. As the why is currently beyond the scope of our scientific testability, they couldn't demonstrate that arbitrary standard false.