All of our known physical laws including GR (which have so far assumed spacetime to be smooth and quite flat) breakdown at singularities due to the infinite curvature of spacetime. Hence, we say, "One second after the big bang..., An hour after the big bang..., etc." Because, we simply don't know what happened at the instant of big bang. If there were events before the big bang, we cannot use those events to determine what happened in its future, because our laws don't work at singularities.
As far as we know, there haven't been a single consequence of the events before the big bang. So, it doesn't make any sense to talk about those events. Since they're of no use, we should probably throw them out of our model of universe and declare that time should've been created since the big bang.