Because the Catholic Church has examined and explored the issue, and she is the one whom Christ promised would be led by the Holy Spirit.
I know you are annoyed, but the next question is, how do you KNOW it is the Catholic Church, those in communion with the Bishop of Rome, is this one institution, only and exclusively, to whom Christ made this promise?
The entirety of the patriarchates of the east consider, and have considered since the undivided Church, that 3Macc is canonical. How do you know the Catholic Church is right?
Now, how about YOU answering the question without just asking another question?
I know that, without question in the undivided Church, certain books remained uncontested and universally affirmed. I know without question that there are certain books that are accepted today universally as canonical, though have been subject to dispute throughout history. I know without doubt that the are certain books that some accept as canonical, while others do not, and there are some that are universally rejected in terms of canonicity.
With that in mind, our communion uses these books in that light. The light of the history of the Church. Yes. The Church, because it is the Church that has the teaching authority, and makes the determination. That doesn’t violate sola scriptura because scripture gives that authority to the Church.
As the Church is divided, and has been for a millennium, it should not be surprising that different communions have made different prayerful judgements as to which books do and do not rise to the level of others. As Lutherans, we hold that the DC’s are not at the same level as canon, but that they are good and worthy of reading, and I would add of great worth to the Church.
The Catholic Church, those in communion with the Bishop of Rome consider them to be canonical and inspired, as does Orthodoxy, who additionally consider other books as canonical.
How do I know Tobit is not canon, or 3Macc is not canon, or The Prayer of Manasseh is not canon? I frankly don’t, because I am not the Church, and I am not the Holy Spirit. I simply pray that I, as a Christian, respond as best I can to the Savior’s grace and call, and ask forgiveness where I am wrong.
Jon