Let's grant that this Necessary Being exists in some possible world, in the same way as unicorns. Why should that be our world? How can you know?
At the age of five, Skagra decided emphatically that God did not exist. This revelation tends to make most people in the universe who have it react in one of two ways - with relief or with despair. Only Skagra responded to it by thinking, 'Wait a second. That means there's a situation vacant.'