Because learning written English isn't easy when the sounds it represents are not part of your sensory experience. It's more like learning Egyptian hieroglyphic

Because learning written English isn't easy when the sounds it represents are not part of your sensory experience. It's more like learning Egyptian hieroglyphics. Plus many parents either can't or won't invest the effort/money to make it happen. Lots of parents never even learn ASL to communicate with their own kids. Met a mother in a beginner ASL class finally deciding to learn enough to communicate with her 20 year old daughter (Who was born deaf and illiterate FTR). It broke my heart when I met the daughter and found out she basically had no exposure to other signers too. Deaf all her life and she didn't have a name-sign. There's "cognitive impairment" and then there's "nobody bothered to communicate enough for a kid to build communication skills."