Why did God allow more books?

Every week, I answer a common question related to the end times. Today's question is about why the New Testament was added to the Old Testament if Deuteronomy forbids adding to God's Word, and why books like Enoch and Jasher were left out of the Bible.

Question:

Deuteronomy 4:2 and 12:32 explicitly say not to add to God’s Word. So why was the New Testament added to the Old Testament? And why are some books — like the Book of Enoch or the Book of Jasher — left out of the Bible entirely?