Here's why:
* Sign language has a long history. Various forms of sign language have been used for centuries, even millennia, by deaf communities around the world.
* ASL (American Sign Language) was established long before 1942. While ASL has evolved over time, its roots are firmly planted in the 18th century.
* Other sign languages existed worldwide. Many cultures had their own forms of sign language, some with even longer histories than ASL.
While the specific forms and vocabulary of sign languages may have changed over time, the concept and practice of sign language existed well before 1942.