But what of it? If they posit physical existence prior to the "big bang," then they believe in an eternal universe. What else would you call pre-big bang physicality?
Perhaps the atheists have a different definition of "universe." Perhaps for them it only refers to this particular shape or suspension of physical reality. For me "universe" means "everything" (or perhaps more correctly "everything phyisical"). If there were physical phenomena prior to the "big bang" and thus no creation from nothing, the universe is eternal. If they're satisfied to just see "how it got to be so pretty" that's their problem. How did the physicality come to be in the first place? That's my question.
More succinctly and even less eloquently: how is there not nothing?
Assume that the physical universe is self-organizing for argument's sake - that still doesn't mean that it is self-generating.