Because you or anyone is most likely to search for relatively popular books. So those books will have a multiple copies. But for every popular book, there are many unpopular, but still useful books, that only have a single copy.

To be fair for textbooks at least I often see several results but often of different editions (1x edition 1, 2x edition 2, 1x edition 3 etc.). In some cases I think it's worthwhile keeping the different additions around, unless it becomes a huge burden.

Usually the different results have meaningful differences - often times different edition or translator etc

In my experience it's different editions or mirrors.