The term "pen knife" historically referred to a small folding knife that was commonly used for various tasks, including sharpening quill pens for writing. While pencils have been widely used for writing since their invention, the term "pen knife" has persisted to refer to a specific type of folding knife.
The reason the term "pen knife" has not been changed to "pencil knife" is likely due to the historical context and the evolution of language. The term "pen knife" has been in use for centuries and has become established as the common name for this type of folding knife. It has become part of the knife lexicon and is understood by many people.
Additionally, pen knives are not exclusively used for sharpening writing tools. They have multiple purposes and can be used for tasks such as cutting, carving, or general utility work. Therefore, the term "pen knife" is still applicable and encompasses the range of tasks for which these knives can be used.
While it is true that pencils are now the predominant writing tool, renaming "pen knives" as "pencil knives" might cause confusion among those familiar with the traditional terminology. It's more practical to retain the established name and use it to refer to the folding knives that have been known as pen knives for centuries, regardless of the writing instrument being used.