Answer: Because according to the science of textual criticism, the Bible is the best preserved and accurately attested document of all ancient writings, secular or religious!

Most people don't realize that we have no originals of ancient documents; we have only copies. That's where the science of textual criticism comes in. To determine the authenticity and accuracy of an ancient document, the science of textual criticism looks at the number and quality of copies (in whole or in part), the date of the copies (with those closest to the date of the original having the most value), and how the copies compare to each other (variants etc.).

There are over 5000 original language manuscripts of the New Testament section of the Bible. One of them is a piece of John's Gospel dated to within 10 or 15 years of the original. In terms of the number of manuscripts, the next closest document is Homer's Iliad with fewer than 650, some quite fragmentary, and with a thousand year gap between the copies and the original.

In "ancient times" like the 1960's, some of us took Latin in high school. Caesar's Gallic Wars was a staple. We never questioned whether what we were studying was what Caesar actually wrote, yet there are only about 10 good manuscripts of the work and they were written nearly 1000 years after the original!

That covers the New Testament but what about the Old Testament section of the Bible? The earliest copies we have in Hebrew are dated around 900 A.D., which is nearly 1300 years after it was supposedly finished. How can we say the Old Testament we have today faithfully represents the text as originally written?  We might begin by citing the care of the copyists counting as they did their work. The number of times each letter of the alphabet occurred in each book, the number of verses in each book, the number of words, etc. Anything that could be counted to insure accuracy was counted. But beyond the mention of the scrupulous copyists, The Dead Sea Scrolls supply the definitive answer.

The scrolls were discovered in 1947 when a Bedouin shepherd boy, searching for a lost goat, threw a stone into a hole on a cliff and heard the sound of shattering pottery. Investigating, he found a cave with several large jars containing leather scrolls wrapped in linen cloth and carefully sealed in jars. The scrolls had been preserved in excellent condition for nearly 1900 years} One of the scrolls found contains a complete manuscript of the Old Testament Book of Isaiah in Hebrew and is dated at 125 B.C. Here is an example of the accuracy of the copyists over the 1000 year period from 125 B.C. to ~900 A.D. (the date of our earliest Hebrew manuscript before the discovery of the scrolls): When comparing, for example, Isaiah chapter 53 of the 900 A.D. text with Isaiah 53 of the Dead Sea Scrolls (125 B.C.), there is only one word of three letters in question after 1000 years of transmission - and that word does not at all change the meaning of the passage (Geisler and Nix, A General Introduction to the Bible).