The way you look at something affects what you see. If you read the Bible already accepting certain ideas as true, your reading will produce a certain result. If you already accept the age of the earth as really old, and the Bible clearly says something else, you have a problem. Both ideas can’t be true. What happens when you want to believe that the Bible is true and your public school educated science teacher is right about being the descendents of pond scum and mutant monkeys? You have a problem of logic. In order to resolve that problem you invent an allegorical reading where no allegory existed.
The Bible does contain allegories. It is very clear when it uses them, such as, “he spake to them in parables”. In the Old Testament there was a hand full of times that they were used to communicate a point. It is clear from a plain reading of the text which portion was literal and which was figurative.
In order to believe something is an allegory, you have to have a reason for that belief. If the reason is in the text itself, then you have a good argument for believing that’s what the author intended. If there isn’t a reason in the text, you have to ask yourself why you think what you’re reading is not what it claims.