Because it’s some people’s native language and for those people English (or whatever is the spoken/written language of the land) is their second language. Sign languages aren’t using hands to communicate in their original language, those do exist like ESL (English as a Signed Language) but the Deaf in America and England don’t use ESL, we use ASL (American Sign Language) and BSL (British Sign Language) respectively. These are very different languages from each other and ESL. They don’t even share fingerspelling alphabets.

Captions are amazing for the hard of hearing and late deafened, especially since many children such as myself who grew up hard of hearing were denied sign. But it’s my language by right and I was denied it as a native language. It’s natural for face to face communication in a way writing isn’t and it’s also a cultural language. A Deaf five year old can understand the news broadcast in sign language just as well as a hearing one can understand the spoken one.