Why did G-d create Mormons? So one sect of Christians would know how Jews feel.
So, if the Mormons say they have a new revelation, and you the (Catholic, Baptist) are therefor incomplete without it, after all, G-d didn’t come down and address the millions assembled in upstate NY to give His new message, right? If you don’t believe the Mormons’ claim, why on earth would a Jew believe your fantasies?
I could say something similar to the Mormon as I could to the Muslim as both make a similar claim about the truth being corrupted that a new revelation was necessary.
This is not a problem that Christians generally have. Even those who have adopted a form of extra-Scripture traditionalism of their own do not maintain that the Gospel and Hebrew Scriptures has been either disrupted or is somehow fundamentally altered.
As for traditions, there is a difference between Scripture and traditions that have been added to it, often long after the fact. Indeed, to this day there is a whole branch of Judaism known as the Karaites that dispute the propriety of writing down and subsequently elevating the traditions.