Why is it when someone loans someone something that isn't returned it's a civil matter? But if someone doesn't return a library book it's criminal?

Why is it when someone loans someone else something and they don't return it then it's said to be a civil matter? However when someone borrows something from a library such as a book a DVD CD etc etc, then it's criminal?

Why is it when someone loans someone something that isn't returned it's a civil matter? But if someone doesn't return a library book it's criminal?
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