Do you know how good things come in small packages?
Well, God comes only in one package.
It is not about one God vs many Gods. It's just that God is something 'different', unlike anything else. Things which are most valuable in this world, are those which are one and unique.
To believe in other Gods or that there are many Gods, then there must be one God who is greater than all those others and who is 'different' in many ways and aspects.
You have that same thing in Hinduism, there are other lesser Gods and then there is Brahman, who is superior to everything and everyone in every way.
We don't believe that God is within everything because God is the First and the Last. No beginning and no end. He is limitless and nothing is comparable to him. If you have that information and then put limits on God or his eternal attributes then that contradicts itself. If God has no limits and is not bound by his creation, then how can that which is limitless be confined to a human body among other things?