For the same reason I think the reference in Revelation 12:9 is metaphorical.I am not doubting the account given in Genesis. I just think your intepretation of it is seriously flawed.Why engage in such a hypothetical? What purpose would that serve? But if you are interested in such things, try this hypothetical; given that the big bang is the actual method that God used to create the universe, and given the modern scientific knowledge used to explain it today, how might God have described that event to the ancient Hebrew?
So let's extrapolate the metaphor a bit, and I assume you have no objection to this.
There was no real serpent, its a metaphor for Satan
There was no real fruit, its a metaphor for the all present change to do something against God's will
There was no real tree, its a metaphor for the event that presents us the option to do something evil.
There was no real garden, its a metaphor for an environment in which God takes prime position
There was no real Satan, its a metaphor for our inner desire to do something evil
There was no real Eve and Adam, its a metaphor for the human race
Now all these come from the same small chapter in the Bible telling us about the Fall.
One player however I have not listed, but logically if all subjects in the chapter are metaphors, than the last one quite possibly is too.
There was no real God, its a metaphor for the good we people can do to ourselves or those around us.
My question is: If you indeed take one part of the same event and declare it a metaphor, then by what reason would you argue the other parts are not?
Where do you draw the line, and why do you draw the line where you draw it?
Finally your question how God could have described the big bang to the Hebrews? Very simple, just say what happened.
Genesis 1:1 (4WD style)
1: In the beginning God initiated the existence of heaven and earth
2: Now there was nothing and the Spirit of God looked at the nothing
3: And God said "Let there be a speck containing all the building blocks of the universe". And there was a speck. And God looked at the speck and saw it was good.
4: And God said "Let the speck explode and form the universe". And the speck exploded. And God looked at the explosion and saw it was a good explosion.
5: And God said "Let the universe bring forth all living things over time". And living things evolved from the universe.
6: And God rested from His work for a while
7: And God saw that the universe had brought forth a living being worthy of salvation
8: And God gave a soul to the worthy being and called them "humans"
9: And God saw that all that had happened was good, and He took a step back to let nature run it's course.
You think the ancient Hebrews could not have understood such a story?
So now my challenge to you. Lets assume the accounts in Genesis as they are written down are literal...what could God have done to make it more clear to you that it was literal?
Would you have preferred an additional verse to Genesis chapter 1 saying something like:
32: "The above really, really, really happened this way, literally"