We are not amending the Scriptures because the Scriptures did not come before our religion. Our religion and culture came first. The Scriptures were written by Jews who were descendants of Jews who had been worshipping YHWH for millennia before the books of Scripture were composed and settled into the form you know today.
Judaism is not based on Scripture because Judaism had to exist first. The Bible did not write itself of fall from the sky as you have it today. Jews wrote it. Jews came into existence not because they had the Scriptures to study and base their religion on. On the contrary, Jews based the Scriptures on their own beliefs long after their religion had started and long after they had settled in Israel. It is the testimony of their convictions, not the basis for them.
The Hebrew Scriptures only have their current status in Judaism because Jews picked them out from the myriad of our many compositions to make up the Tanakh. Jewish authorities, our rabbis and great sages, decided what books were sacred and which were not. They would have no authority unless we made it so. They did not authorize or write or canonize themselves. Someone with authority from outside the Bible had to do it, someone and something with more authority than Scripture.
Jews base their religion on a series of claimed theophanies that our ancestors claim they experienced. It is on the basis of these, especially the Great Theophany at Sinai, that our religion stands. That came before even the Ten Commandments were written by God. Those theophanies are the "proof" our ancestors had of YHWH, not Scriptures.
God did not write us Scriptures and hand them out to Abraham, Issac, and Jacob and tell them to follow what was written. God did not hand Moses the first five books of the Bible and tell him to start a religion based on it. We Jews did not stumble over these books or find them hidden in the ground like Joseph Smith and then decided to base on religion on what was written.
God revealed our religion to Abraham, Sarah, Issac, Rebecca, Jacob, Leah and Rachel, to Moses and the prophets. We didn't learn it from a book. We wrote the book. We invented it and based it on what was revealed to us. The Scriptures have never replaced or will ever be seen as greater than the theophanies experienced by our ancestors. They are a testimony to it, part of the deposit of that revelation, but not the final or primary authority. The Scriptures would not have authority unless we believed our religion had authority.
In the light of this we interpret our history and our religion and its meaning based on human growth and achievement. The words in Scripture, from our God, yes, are however not seen as static. We are not called to return to the snapshots in time preserved in it's pages but the call to our hearts placed there by God to which the texts give witness.
I understand and appreciate that you hold the written books as the ultimate authority. Religious Jews feel they have something greater. God didn't give us Jews an inspired book in order to make us his people. God made us his people so we could write an inspired book.
If God made us a nation and inspired my people to write a book you feel must be obeyed, why do you find it appropriate to question the authors who God used to compose it? Do not authors have the right to explain their own work?
I know you probably don't agree and won't ever, even if the heavens shook and told you so. And I don't claim that everything here represents my personal convictions as a Jew either, but it is the general way we as a people approach the Scriptures and why we have a progressive theology not bound by a snapshot in time written in scrolls.
Your claim may be that your religious beliefs are based on the Bible. But we claim that our religion is based on meeting YHWH face-to-face, and that the Bible is based on what we believe. That is very, very different from where you are. Not more superior (and not meant to claim that they are better than other views), but definitely different in how we use Scripture. Be happy with what you have. We cannot change our view on Scripture because our religion is not based on it like yours. Our religion came from a different source than just writings.