Have you never heard of a rhetorical question? I asked because, although there are some that doubt the authorship, I have faith to believe they were the inspired writings of St. Paul. The real question is why does anyone consider them to be a part of the Bible.
Actually, I can usually pick up on what is asked "rhetorically" versus what gets asked all the time as if no answer that has ever been given is acceptable. If the "real" question is why anyone considers the books of the Bible to BE part of the Bible, I would say the answer is the same as to why the Old Testament books were considered part of the Bible. God made sure that what He said was written down, that it was reliable, that it changed lives, that what He said came to pass AS He said it would, that God's hand-selected prophets made known to God's people what He wanted them to hear and know, that God made Himself known to His people through these prophets as well as through miraculous manifestations of signs and wonders that followed them. The writings, by their very nature AS divinely-inspired, were acknowledged as the Word of God for thousands of years.
Far from God just leaving it up to a group of men to shuffle around a stack of Scripture wannabes and decide for themselves what was and was not "God's word", He directed the process, personally. The Jewish people collected the writings of Moses and the Prophets, their theologians wrote commentaries on the various books, synagogues kept copies of these books and young Hebrew boys learned the Hebrew tongue so that they could read these writings from Jehovah. So was the case with the New Testament. The Apostles and their disciples wrote under the inspiration of the Holy Spirit - God leading them in what was said - and these writings were circulated amongst the assemblies. What the disciples and Apostles taught was what Jesus taught them - these also being written down to preserve the authority and authenticity of the doctrine. The very fact that the writings were authenticated BY the Apostles made them immediately and universally accepted as Holy Scripture and they were mutually supportive of each other in doctrine. They shared a core doctrine of truth and the early church knew personally many of the writers as well as these writers knowing each other and recognizing their shared faith - almost like a "peer review". God made sure that these writings were safeguarded from error so that His people would have all they needed for doctrine, reproof, correction, so that the man of God may be complete, thoroughly furnished for all the work of God.
From http://hipandthigh.blogspot.com/2005/07/liberals-kjv-onlyists-and-inerrancy.html we learn:
God is the sovereign Lord of all things, and that most definitely includes His revelation. If He has the absolute authority to create everything that exists, govern nations, raise up and put down kings and their societies, then God can certainly govern the accuracy of the details recorded by the writers of scripture. Peter confirms God's sovereign hand in recording scripture when he writes, for the prophecy came not in old time by the will of man: but holy men of God spoke as they were moved by the Holy Spirit (2 Peter 1:21).
God safeguards the transmission of His written revelation through the thousands of copies handwritten by His people, both during the time of the OT and the time of the NT. The body of textual evidence for the Bible is compiled from hundreds upon thousands of entire manuscripts, portions of books, fragments of books, translations into various languages, historical citations and so forth making it the most attested piece of ancient literature ever written.
God protected His revelation by allowing the biblical documents to literally "explode" across the ancient world at different times and in different locations through its many copies. In this way, His revelation was safeguarded from any one group gathering up the scriptures and altering the content. Within the first 300 years of the Christian Church, these copies of scriptures were so far flung there could be no organized effort to genuinely corrupt the Bible. The one side effect, however, is the presence of minor copying errors that could always be corrected.
So, to answer your question of why anyone should consider the books that are in the Bible as belonging in the Bible, I think we can credit the church, the Apostles and disciples and the power of the truth of God to speak to the heart of all those who sincerely seek Him. When I read the Bible, I know it is God speaking to me - His love letter - and that is NOT something I get from any other writings.