If you are born Deaf, you will not have the opportunity to natively acquire a spoken language. Our brains are not designed to acquire spoken language through writing -- and our spoken languages are designed to be acquired through listening, not reading.
It's easy to think that a Deaf signer living in America is a native speaker of English, but they very well may not be, having never heard the sounds and therefore been unable to internalize all of the many, many necessary grammatical and morphological rules of the language.
Expecting all Deaf people to simply use written English when dealing with a computer is like expecting all Americans to use Spanish to interact with their computers -- technically possible (Spanish is a perfectly good language, and relatively accessible starting even from grade school here), but ignores the human reality that people want (need) to speak their own language.
There are not yet standard ways to write sign languages. Partly this is because of difficulties in translating the medium to text -- unlike speech, sign language production is not entirely linear, in addition to the fact that your hands can be articulated comprehensibly in much more sophisticated ways than your mouth can.