I’m quite certain it’s a carry-over from the ‘middle’ ages. Before the printing press, books would take an extreme amount of resources to create. You had to get the leather from a calf or ten (IIRC one calf has enough skin for two pages), which would almost invariably be bought from multiple villages within a lords domain. Then you had to prepare it; the reason calves were used was because their skin would be relatively undamaged from bug bites (which would became holes during prepartion) and would have somewhat less hair on it. Then you would have to have a scribe actually write in it and have it “illuminated” by other craftsmen (making it look pretty by applying gold leaf to the cover and/or the first letters of the chapter). This would all take years of labour as well.

By the end, a finished, illuminated book would likely cost more than the entire building it’s stored in, hence the chained libraries.