Answering the original question: Because the Church has defined the Bible as Scripture.
If the books are taken first, not as scripture, but at least some form of historical witness to what was going on in the first century movement, and given other historical testimony and the explosion of Christianity, and taking this all in the context in which I’m rationally confident that there is a God who is at least capable of interacting with the natural order, I’m led to believe that there was a historical Jesus Christ who claimed rather extraordinary things about himself. And I’m led to believe that he was crucified, resurrected, and established an authoritative, visible Church who had the authority to act in his name, set disciplines, teach on faith and morals, etc… and so I trust the Church’s authority in declaring the books of the Bible scripture.
Is this circular reasoning? No. The following would be circular. “I believe in the Bible because it’s the Word of God and so is absolutely true. I know it’s the Word of God because the Bible says so and is absolutely true.”
I’m not starting by taking the New Testament books as absolute truth or the Word of God. I’m taking those books, along with other early Christian works and history, as evidence of some type of historical witness (supported by the quantity of them), reliable insofar as such ancient works can be reliable. I then find the most rational explanation for the explosion of testimony and other actions to be that the primary events contained in them are true. It’s only after coming to this, and belief that the most rational explanation is that the establishment of a Church is also true, that I accept the Church’s position that these are more than just a regular witness to some type of history, but as Scripture.
Hopefully I’ve been at least a little coherent in this admittedly rushed post.
EDIT: Actually, it seems porthos has already explained this much better than I have.