Because a lot of people think he may have existed, and probably for the same sorts of reasons some other misguided folk might be interested in the improbable activities of the likes of J Christ esq.... Because they are incapable of discerning the difference between fact and fantasy.
I think we should care because sjc does and because people care to read it, but please do tell us more about this fascinating Abdul Alhazred-character (seems like some archetype pre-renaissance arab scientist)