Hello ketsune23...
Here is a section from TruthBook's topical study on the fifth epochal revelation (The Urantia Book) that might help you to reframe your understanding of the issue that you are presenting here:
Quote:
Epochal revelation builds upon itself; each revelation advances the ideas of the last revelation
Adam and Eve could came only as a result of the advances made from the Planetary Prince's reign.
On our world, Melchizedek came after Adam and Eve (not a normal progression) because of the default of the Edenic pair, and the ensuing loss of the one-God concept among the races of mankind.
Melchizedek prepared the way for Jesus, who revealed the one-God concept - again in the form of a human being.
The Urantia Papers came about 2000 years after Jesus, again revealing the loving Father that Jesus had personified for us; The Urantia Book also contains the complete restatement of the life and teachings of Jesus, filling in the gaps in Scripture, which was the only real earthly record of his amazing life up until then.
On reading this, maybe you can see why there is so much emphasis and quotation from the Bible. It is all that the world has had as a documentation of Jesus' life and teachings. It is the standard of reference for most, if not all, Christians - really, the whole world. The Urantia Book harmonizes the Bible with the new revelation of Jesus' life by clarifying, sorting, and correcting.
Jesus did a lot of that, too. The Scripture of his day was looked upon as being inerrant, but he strove to clarify it for his followers. He strove to emphasize the best of the Scripture, while de-emphasizing the parts that were not consistent with his revelation of the Father.
The other books that you mention are not part of any other epochal revelation, but the Bible is.
You can have a better look at that topical study by going here:
https://truthbook.com/urantia-book/the-fifth-epochal-revelationAnd here is that wonderful discourse of Jesus to Nathaniel about the truth of Scripture:
https://truthbook.com/urantia-book/paper-159-the-decapolis-tour#U159_4_1Finally, continue down in that paper to read about the positive nature of Jesus' religion that expands upon this idea:
https://truthbook.com/urantia-book/paper-159-the-decapolis-tour#U159_5_1