I don't assume there was a before the big bang, there is such a thing as reason. Reason tells me that before this universe came into being, eternal existence/God was (and is and will be). Also I don't assume the reality represented by the name God exists, it is a fact. And the fact that there is no objective evidence to support your claim there was no before, and not a clue as to how and why the universe exists, is icing on the top.But a mathematical theory based on a theory that has been verified by observation is. General relativity has been extensively tested. The BB is what you get when you apply GR to the universe as a whole.
Why? It is a theory about how the universe is *after* the beginning.
You don't need a theory of planet formation to be able to describe the orbits of the planets.
And that would have no effect on the BB description. If you understood what the BB model *actually* says, then you would know that.
Actually, we *do* have this, but within the universe. Quantum fluctuations do exactly this. And, in some models (extensions of the BB model), the BB is produced by exactly such quantum fluctuations.
But, I would point out, once again, that you are *assuming* that there was a 'before the BB'. And you do this without any good reason for that assumption.