As do the Dead Sea Scrolls, at least in regard to any places where their text does not match the established canon.
And, where they are valuable to subsequent translatory updates, they have been used. That's partly why more modern translations differ from earlier ones. The question isn't with regard to supporting manuscripts, but to brand-new material.
So... when mistakes are found, you don't correct them? That approach doesn't make the canon accurate, it only makes it authoritatively inaccurate.
Mistakes, omissions, blurry translations, etc. are all "corrected" as they become apparent. We're not talking about the process of translation. We're talking about the inclusion of completely new material. That cannot happen, because the Canon is closed. The new material can be "added" as corrollary, but not as part of the "standard."
Because the argument "Tradition is Tradition, and nothing else is acceptable" appeals only to consistency, not accuracy or any sort of idea of "truth".
Because it is consistency in areas of origin, authorship, intended audience, literary style, etc. that verify the documents as authentic. It is that authenticity that is sought, not factual consistency of the text. Preserving the tradition is more important than trying to make facts line up. Even a cursory glance at the historical books of the OT will attest to that. That's because the impetus for studying the Bible is to gain insight into the lore -- the story -- of God's people, not to master factual content.
When truth is garnered from scripture, it is largely in the sense of "that's what God's people have believed." There is always a theological push to find relevancy for modern times in the ancient theological views preserved in scripture. In some sense, "truth is truth." In another sense, our perspective of what is true must be tweaked as new understanding and insights come to light.
Why wouldn't you evaluate the Book of Mormon on its own merits rather than saying "sorry, time's up! You should have submitted this sooner"?
It haas been. it adds nothing of significance to the existing books of the Canon.