Actually, I did not say "why do we need one?". That implies subscription to the belief that there is no answer, or at best apathy towards the question, neither of which are attributes I have.
I said the big bang didn't really need an explanation. Assuming we believe it happened, that's all we really need to know as far as the rest of the universe goes. Insofar as we understand the universe, what happened after point zero was independent of what happened before it.
This lack of explanation is not unique to physics - there is no science that describes events outside this universe. Obviously that doesn't mean that there wasn't 'a cause'.
The creationist approach would be to suggest some made-up explanation, surely?