Because their are so many different God`s Worshiped in the world today and none of them have made themselves known to us in this 21st Century for all of mankind to see and verify .
All of the Gods worshiped today are from hundreds and even thousands of years ago derived from ancient writings and texts in languages that are no longer in use today.
That of course brings up problems of another sort translating an ancient language into its modern day equivalent.
And nobody is in total agreement that such translations are accurate or correct anyway.
And their is no God coming to the rescue of translators to say which is the right interpretation. They are mute.
I`m not just talking about Christianity/ Judaism either, the same goes for all non Christian religions too.
Its a total cop out to say one must have faith to believe in God
"Faith, belief in religious doctrines divine truth without proof" and no verification from any God Christian/Islam/Judaism or otherwise .