Here's why:
* No innate knowledge: Unlike spoken language, which has some innate foundations in the human brain, sign language isn't something we are born knowing.
* Cultural variation: Different sign languages exist across the world, each with its own unique signs, grammar, and structure. This wouldn't be possible if sign language was innate.
* Teaching and learning: Sign language is taught and learned, often through formal classes, Deaf communities, or family members.
While there are some arguments that suggest certain aspects of language acquisition, including sign language, might be influenced by innate abilities, it's clear that the vast majority of sign language is learned through exposure and practice.