Not all of Christendom believes all of the Bible literally. The Roman Catholic and Eastern Orthodox, and probably Anglican Churches view it as largely metaphorical and allegorical. Even when I was Christian I did not take it literally. My priest in the Eastern Orthodox Church used to say "what does it matter if Adam and Eve actually existed? The important thing is that we do exist to give glory to God". Yet there are large numbers in various Christian denominations that take the Bible literally. Why? Does taking it as largely allegorical somehow diminish any truths or lessons it holds? Does that make it false?
Using my own Hinduism for example, it's safe to say the overwhelming number of Hindus do not take most of our scriptures or stories literally, specifically the puranas. The Vedas are the exception in that they're generally accepted lock, stock and barrel because they are apauruṣeya (lit. means "not of man", i.e. divinely inspired). But they are not the equivalent of the Bible. The Vedas are hymns, poems, prayers, musings and treatises on theology, ontology and epistemology, and the world, etc. In the Nasadiya Sukta the Rig Veda even questions how creation came about. That said, that we don't take most of our texts literally doesn't diminish their value as being divinely inspired and holding truths.
So why is it so important that the Bible be interpreted literally?