Why? Because the idea that every thing that exists has a cause is as far as we have been able to go in the entirety of human thinking. Our current understanding of physics is what we know. You are implying that there is more to know, that is not known. If it can not be known, it can not be proven, so we have to infer it through its absence in our ability to observe it. So you believe in something you cannot prove to be true, and you do so in order to argue that God must not exist because we can not prove God exists. Start by getting out of your head the idea of God as an old bearded man in the sky - that is the first step. I could from here leave you with the burden of showing through logic why you must necessarily prove your assertion that we do not know enough about physics to positively state that everything has a cause and you would in turn be forced essentially to make the same argument for the existence of this unknown basis for physics as an argument for the existence of God, but I will instead change tack.

So you have to go with the most likely possibility.

If existence is caused by something outside the laws which govern physics...

What is the nature of that thing? What would you call it?