Thank you for the question of copyists of the Bible, “Why did people write bibles thousands of years ago?”
As one reads/studies the topic of canonicity, the copying of the scrolls in the Old Testament and the letters in the New Testament was primarily to preserve the written text.
It must be remembered that in the Old Testament that the priests were entrusted with the Torah and later with the Prophets and of the New Testament epistles (including the Gospels) were written to churches and held by them. The scrolls and letters were copied as major Jewish and Christian centers as Alexandria, Antioch, Rome, Constantinople would want a copy of the Old Testament scrolls or New Testament scrolls. Scrolls and Letters were at times circulated but more often than not it is likely that the scrolls or letters were held in one central place whereby copies can be made of the copy. The lost of a precious scroll or epistle would have been viewed as a catastrophic.
One of the earliest copies of the complete Old Testament in Hebrew that has survived is the Codex Leningrad, dating to about the 13 century. One of the earliest copies of the complete Greek Old Testament is the Septuagint, dating about 3rd BCE. The earliest surviving New Testament is the Codex Sinaiticus about 4th CE.
For a copyist or a group of copyists to copy the whole Old Testament and or the New Testament took great careful effort. One must remember that there were no erasers. The copying of individual scrolls or epistles was so that the believers can read or hear the Word of God.
The Ethiopian eunuch did not buy all the scrolls of the Old Testament. He just purchased one scroll, Isaiah. The priests would be reluctant to give any of the Torah or the Prophet to any person. For him to have purchased one must have met that his standing as a representative of his kingdom allowed him that privilege to buy one.
Prior to the printing press, only the most monumental event would a complete Bible be given to a person perhaps as one’s coronation or one with the means to purchase one. During the Dark Ages and Renaissance of Europe, literacy was between 4–10% whereby only priest and civil servants were able to read and to have access to the scrolls and epistles. It was the priest who read the Bible in Greek and Latin.
Thus for a Jewish or Christian center or large synagogue or church to have access to their sacred writing was a highly treasured possession of the Church. One wanted to see and even touch that scroll or epistle in which that person believes the promises of God.
Imagine if there was only one copy of the United States Constitution, what would that means for Americans if it was destroyed or loss? Having a copy of the Constitution assures the citizens that they can read it for themselves to see if what the priest or pastor declares is true or false.
SUMMARY: Believers want to hear directly from the written Word, than one’s comment or citation of it. It allows one to affirm their belief by reading or hearing the Word themselves.
For more perspectives:
https://www.quora.com/Why-did-people-write-bibles-thousands-of-years-ago