Why should we assume (believe) that the universe spontaneously popped into existence without any cause?
Why should we assume (believe) that the universe has always existed?
Can you show me where I have assumed either statement to be factual? You cannot, because I have not made such an assertion. You, on the other hand, have implied that our universe has a cause. When asked to explain why you believe this, you dodge the question. Are you going to answer my questions now and support your claim with evidence, or are you going to dodge it again?
Incidentally, neither statement is incompatible with the known laws of physics. Modern physics allows for both possibilities. We know about virtual particles that pop into existence out of nothing, and it is not a stretch to hypothesize that spacetime itself could also pop into existence out of nothing. It is also possible that the universe we observe today has always existed in some form, and may also be part of a much larger continuum that scientists call the multiverse. None of these hypotheses require the existence of a supernatural creator that is not bound by natural laws, and that is where your objection seems to stem from.