Several reasons:
1. The Bible starts out IN THE BEGINNING --- not ONCE UPON A TIME.
You are still trying this? All the the other descriptions of creation in the bible that use the phrase 'in the beginning' are full of metaphor, allegory and symbol. It was only six days ago I pointed
these passages out to you, have you forgotten already?
2. We are going to be judged by God's Word; why would It start out with allegory?
Doesn't the bible end with a book full of allegory? The book of Revelation even talks about the last judgement. I wonder how your literalism will stand up to being judged by God's word. Actually the most relevant description of judgement is Paul's description of believers works being judged.
1Cor 3:10
According to the grace of God given to me, like a skilled master builder I laid a foundation, and someone else is building upon it. Let each one take care how he builds upon it.
11 For no one can lay a foundation other than that which is laid, which is Jesus Christ.
12 Now if anyone builds on the foundation with gold, silver, precious stones, wood, hay, straw--
13 each one's work will become manifest, for the Day will disclose it, because it will be revealed by fire, and the fire will test what sort of work each one has done.
14 If the work that anyone has built on the foundation survives, he will receive a reward.
15 If anyone's work is burned up, he will suffer loss, though he himself will be saved, but only as through fire.
Oh dear it is another allegory.
It looks like you literalism will be judged by allegories, you should ask for a different jury.
3. It doesn't make sense to have allegory, unless you have the source of the allegory first.
AV meet Jesus, Jesus, this is AV. Careful you don't want to sneak up on him unannounced with any metaphors or parables.
4. Philo of Alexandria is the one who started the Allegorical Method of interpretation by trying to meld Jewish literal thought with Greek allegory.
And did Paul learn allegory from Philo? Gal 4:24
Now this may be interpreted allegorically: these women are two covenants. One is from Mount Sinai, bearing children for slavery; she is Hagar.
5. Jesus, His disciples, and every [human] author interpreted the books of the Bible literally.
If they didn't would you be able to tell when you interpret it all literally anyway?
John 6:31
Our fathers ate the manna in the wilderness; as it is written, 'He gave them bread from heaven to eat.'"
32 Jesus then said to them, "Truly, truly, I say to you, it was not Moses who gave you the bread from heaven, but my Father gives you the true bread from heaven.
33 For the bread of God is he who comes down from heaven and gives life to the world."
6. In almost every instance, the Bible alerts the reader when a non-literal passage is coming up. Where It doesn't alert the reader, the passage is so allegorical anyway, there's no question (such as the trees talking to each other in Judges 9).
Just an excuse to exclude the passages you cannot deny are allegorical. If there is no question when passages are allegorical, why is the church divided down the middle about taking Jesus literally when he said "this is my body"? Why was Nicodemus confused when Jesus said "you must be born again"?
Anyway why is there 'no question' about a talking trees being allegorical but for most Creationists a talking snake has to be literal? It seem arbitrary to me.
7. One of the Ten Commandments is based off of a literal Genesis 1.
And the same commandment in Deuteronomy is based off a metaphorical description of the Exodus when God rescued the Israelites "With a mighty hand and outstretched arm" Deut 5:15.
8. Lineages are traced all the way through to Genesis 1.
Are these the genealogies Paul keeps telling us to avoid? And Luke describes as 'supposed'?
9. The sentence structure of Genesis 1 is neither allegory, nor poetry, but literal.
Sentence structure cannot be literal or non literal, do you mean narrative? I don't see how that can help you as narratives can be literal or figurative. However Genesis 1 is unlike any other passage I know of in scripture, the nearest I can think of are the repetitive refrains and parallelism of some of the Psalms.