Because God is not a limited object in the universe.
It is a matter of the definition of God – in other words, it is an analytical truth. What are we asking when we ask if God exists? We are asking if there is some being that is a necessary truth, upon which all other contingent truths are based.
The statement “God is not a limited object in the universe” is part of an analytical definition of God; you cannot challenge it, although you can claim that no such Being exists. It is, trivially, true - trivially, because it is just how the disputant is claiming to define a word.