The real question is... what is the advantage of written sign language versus... normal writing? I think a lot of people are confused and think that there is only one universal form of sign language used worldwide [1].
Second problem is that sign language is heavily influenced with corresponding facial expressions, body language, the motion of the hands, even how emphatic the motions are. Trying to approximate what is effectively a SPATIAL language into written glyphs feels like a complete waste of time.
> what is the advantage of written sign language versus... normal writing
If your native language is French, why might you prefer things to be written in French rather than, say, Swahili?
I feel like we might be talking past each other but it is funny that you chose French and Swahili. [1]
The point is that "SIGN LANGUAGE" is idiomatic to the native speaker's tongue. So if you're going to take the time to create a specialized written form of it, you can just write using the native language which can be read by BOTH the Deaf and non-Deaf community.
Deaf people are not magically illiterate.
Creating a written sign language serves no value since it is just a crappier version of the normal written equivalent.
So there's not a lot of value in creating a written form of say the French Sign Language because you can just use French.
Swahili regions have multiple types of sign language including Kenyan Sign Language.
[1]
https://en.wikipedia.org/wiki/French_Sign_Language
https://en.wikipedia.org/wiki/Kenyan_Sign_Language
> The point is that "SIGN LANGUAGE" is idiomatic to the native speaker's tongue.
No, this is not true. French and French Sign Language are totally unrelated languages. Sign languages generally have little to do with the spoken language of the country they’re used in, that’s why for example American Sign Language and British sign language are completely different and not mutually intelligible despite the UK and the US speaking the same language (with only slight differences in accent and vocabulary).