Why do people say there is only one God ?

You can't prove that any "God" exists.

There is no proof that there is only one God, but our faith in that there is but one God, and that the entire universe is part of Him keeps us going another day.

Well, I keep going happily without gods...

I would think the question of this thread was asked back during the heavy proselytizing in the first millenium by many a pagan. I almost believe that something happened to our brain wiring that the concept of one God seems only natural to many, while the concept of many Gods seems silly to many. The Hebrews were laughed at for believing in one God.

Plus, many people that believe in a God think it is silly to believe that no God exists.

It may be futile to argue the whole God concept, since it may have to do with different wiring in our brains?

In general, Judaism views the existence of God as a necessary prerequisite for the existence of the universe.
The existence of the universe is sufficient proof of the existence of God.

One of the primary expressions of Jewish faith, recited twice daily in prayer, is the Shema, which begins
"Hear, Israel: The Lord is our God, The Lord is ONE."

You can't prove that God does not exist

That is also correct, but, what do you mean by 'God'?

Is it an eatable one - suitable for use as food ??
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I highly suggest you do some background research on the roots of monotheism. There are a handful of proposed reasons for the shift from multi-god paganism to single-god paganism -- the most cogent argument that's I've read is that Egyptian (and other) monarchies tended to be associated with one particular deity, so revering all other deities with the same intensity was viewed as regally threatening. It was not so much monotheism at this stage as it was polytheism with a supreme godhead, and it more or less evolved from there. In fact, while proselytizing pagans early Christians often tried to associated their god with whatever the supreme being was in the region, and then pass off the other subordinate deities as "prophets" or "saints" or demi-gods with analogous stations. But, this is just the tip of the iceberg.

In Scandinavia the beliefs in the old gods and the Christian God lived side by side over a couple of hundred years. They even mixed, so the Christian God was accepted amongst the old gods.
Some vikings had already become Christians when in Italy or France were they had seen the beautiful churches which impressed them very much.
The Danish King Harald became christian when the Saxons threatned Denmark and he wanted more protection .... so everybody had to become a christian.
The women liked Christianity - espeicially Mary and the promise to go to Heaven after death. In the Nordic religion the men went to Walhalla where they could celebrate and drink for eternity. The women went to Helheim and could never rise from the dead. They only lived through their children.