This really contains many questions.
Where did they go? Most likely they were destroyed by fire, water, deterioration with time, or just trashed. Texts must be periodically copied to survive for long time.
Some monks in some places did "feel necessary" to copy them, others at different places and times did not. Because this was "pagan literature". The attitude to the pagan literature among Christians (and Muslims) varied, depending on place and time. A book has to be copied MANY times to survive 2 millenia.
Why were so many books discovered during the Renaissance? Because interest to these books resurrected, a DEMAND appeared. Then they started to search and publish the surviving copies, and translating what was previously translated into Arabic. What they found they published. Publishing with a printer press, of course produces much more copies than copying by hand. What they did not find, and did not publish, mostly disappeared. Something was found later and published. (Why and how the interest resurrected is a topic of another long discussion).
What was the pre-Renaissance book trade? There was no books in the modern sense: printing was invented in 16s century, at the time of late Renaissance.
Before that there were manuscripts. They were very expensive. This was a kind of "luxury item". Very few learned people were interested in them. They collected and exchanged them, and copied them on a relatively small scale. Most people, including rich and learned ones had other priorities.
Sometimes, in some places, manuscript were copied on a large scale (in antiquity,
in the places like Alexandria library, and few other places). These collections did not survive.
As an illustration, let me give just one example. The work of Archimedes which is
considered his best (of those surviving) by the present generation, was found
as a single copy in a Christian Orthodox storage in Istanbul after WWI. For those who "preserved" it, it was not worth the parchment it was written on. They erased it, and wrote their religious text over it. These religious texts are of little interest to us, and the erased text was read with difficulty using modern scientific methods. Many other pagan books were "preserved" this way: because some ancient copies were written on expensive parchment, and they did not erase them very carefully.
I suppose I answered all questions.
Currently we experience a transition. From printed paper books to electronic formats. Most printed books will soon become antiquarian items. Certainly those of interest to sufficiently many people will be copied to electronic formats.
Others will not be. Some of these (others) will be destroyed, because our libraries do not have "enough" storage space, others will rot in the basements.
At some later time, the interests of people may change. And books destroyed by the present generation will be searched and copied again, and some parts will be lost etc.