The system is based upon cards and cardboard/paper slips: it is a low-technology approach yet proven over 100 years to be robust and scalable to work even in large libraries.
Training can be completed in a small number of hours, and the cost of implementation is mostly centred on labour.
Typically, a small number of 'queries' will mount up, cases where something goes awry because a card or ticket is misdirected in some way.
A tray of cards dropped on the floor could prove catastrophic and require a substantial amount of time to re-sort.
The advent of technology within libraries, such as barcode scanners, eliminated card-based systems.