One word: Genius. Nobody creates covers illustrations like Jacobus. I saw recent covers of modern Goosebumps books that were made by other artists and no one of those covers is at Tim's level...he has something special. The other covers from other artists aren't scary at all, while Jacobus ones always had at least something strange and ominous, even when you didn't see any monster drawn on the covers.
Thank you very much for commenting, I find your comment very interesting, I think you have defined Tim Jacobus' work in a very good way, even better than I did in the video