I doubt the Golem of Prague existed but why wasn’t he mentioned by… isn’t really such a question?
According to all version of the Golem story it’s existence was a strict secret at the time. Therefore it would make sense for people not to have heard of it. That would also explain why there is no written reference to it prior to the 18th century.
On another note while I doubt the Golem existed I also don’t believe that the origins of the legend are some obscure 19th century author . It could not have become such a widespread legend on the basis of some author who 99.99999% of people ever heard of in any context other than the claim that he was the one to make up the story