But I found that it's possible to request books like that through other libraries, since most libraries have agreements with other libraries to allow interlibrary loans
Typically, a public library is legally prevented from spending tax dollars to provide services outside the taxing area. This is a pretty reasonable limitation, but it does prevent the actual mechanics of operating interlibrary loans. It doesn’t really cost a lot of money - a van and a few part-time drivers can cover a pretty wide geographic area. But that money has to come from somewhere else. That’s where those federal grants came into play.