Obviously because it alludes at the "books of secret knowledge" of yore, hard to find books, compiled by masters of some domain and filled with useful knowledge.
It's like calling your Chinese restaurant "Golden Palace". It's not golden and its not a palace. It's just meant to convey "typical chinese", "luxury", etc.
I chalk it up to wanting an attractive title. The information is not actually secret in that the reader or writer is bound to keep the information secret. Rather, the knowledge was previously difficult (to some degree) to come by, and is now co-located, thus lifting the shroud of secrecy (by obscurity).
Not everything has to be "clickbait". Let's keep it to those who make money off of news posts, and blatant examples like "You won't believe what Lindsay Lohan did...".
It's like calling your Chinese restaurant "Golden Palace". It's not golden and its not a palace. It's just meant to convey "typical chinese", "luxury", etc.