Just because we haven't observed something popping out of nothing doesn't mean it can't happen. Maybe the rules are different for the universe as a whole.

I think I can understand the heart of your objection. One reason that we assume that things can't pop out of nothing is that our experience tells us this doesn't happen, so it's an example of induction, inferring from the specific to the general. And if it's just induction, maybe there's nothing logically contradictory about that happening. But there are two things to be said about that.

First, just because it might not be logically contradictory for that to happen, that doesn't mean it's a good explanation; we'd still need some good reasons to think that it happened that way. For example, imagine you walk into your apartment and you see your roommate hunched over a plate. He looks up and he has chocolate smeared on his face and crumbs on his jacket. He's been eating the double chocolate cupcake that your mom gave you for your birthday! You're about to accuse him and he says "Wait, wait, it's not what you think! Isn't it possible that while I was walking through the kitchen, the cupcake spontaneously appeared in my mouth?" Would you believe him? Of course not! He'd have to give some evidence that he really didn't just take the cupcake from the fridge and eat it. The point is that we have no evidence of anything physical spontaneously happening with truly no explanation, and it seems absurd, even if you argue it's not technically impossible.

Second, the first premise, that everything that begins to exists has a cause, goes deeper than just something from nothing. When you reject this premise, it's not just saying that there was no stuff and suddenly there is stuff, violating the conservation of matter and energy. You're actually claiming something way more extreme than that: that something can begin with literally no cause at all. Not just an unknown cause, but absolutely no cause or explanation.

Because this argument is okay with leaving the outside cause unknown, even if it does have similarities to God. But to say with confidence that there is just no cause, I think goes way beyond our ability to know, and contradicts our intuitions and observations that everything does have an explanation. Don't you think it's more reasonable to say the universe might have an explanation rather than needing no explanation?