If God can always exist or come from nothing, why can’t the entire physical universe always exist or come from nothing?
You have a real apples to oranges problem in your question. The universe is a material, measurable, NATURAL phenomenon. Materialistic items existing in the observable dimensions of reality are at the mercy of physical laws. One of the biggies in physical laws is the law of causation.
Trying to apply those laws to the Supreme Being -- an (list your 'omnis' here) God breaks down and does'nt work. Theists do not and will not rank God with a natural, material phenomenon. A being that is beyond natural, completely apart from natural, "supernatural" if you will, does not answer to the mundane physical laws that govern waves, particles, and forces.
So to say that 'God always exists' is not on the same level as saying 'these particles have always existed' or 'this wave had no origin'. I hope that puts a little meat into the uncaused cause and the prime mover argument for you.
As a point of fact, it seems that the natural universe may not be measurable or observable at the quantum level of modern physics.
I agree that there is no reason to require that God have a physical existance in order to believe. However my original objection is to the use of observable, physical things, no matter how complex or mysterious, as evidence of a supernatural creator. That objection seems to me to be strengthened if you put God outside of physical existance.
Nobody ever mentions what seems to me to be the strongest evidence for the existance of God, that simply being the fact that there is anything at all. A void is perfect and complete, why is it cluttered up with things? Somebody must have had an idea.
I still utterly reject the idea that any physical thing offers evidence of God. Explaining complexity by postulating greater complexity as a cause just will not fit into my mind.