Those who have any dealings with the Necronomicon usually come to an unpleasant end, and Alhazred was no exception: legend has it that crowds saw him being devoured alive in a public square in Mecca. The being or beings that did the devouring remained invisible throughout.
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.