
The reason for this is that as a mind-reader, he always knows everything about everyone with Mashiro being the only person he has found who he can't read. The main character Kuroi, who possesses mind reading and hypnosis powers, will take extensive steps to ensure no-one prods or reveals anything in Mashiro's mind or past.
A strange In-Universe example comes from Thou Shalt Not Die. He still offers her to tell everything if she insists, but Robin decides to take his advice and does not ask further.
So figuring it out by her own may give her another conclusion than the Roger Pirates got. and the Void Century, and while he knows all about it, he advises her to find it out for herself, since the Roger Pirates lacked the archaeological background to fully grasp the history.
Similarly, Robin is about to ask Rayleigh about the Will of D. Clearly, Luffy is big on spoiler-warnings! When Usopp asks Rayleigh about the One Piece, however, Luffy angrily stops him from completing his question, declaring that knowing it would take all the adventure out of SEEKING the One Piece - in fact, if he found out in such a boring way, he'd immediately quit becoming Pirate King. The Straw Hat Crew eventually encounter perhaps the only living person who would actually KNOW, however - Silvers Rayleigh, who was Gold Roger's First Mate. There are many hints that it is far more than just your traditional pile of gold and gems - such as Gold Roger's apparent involvement with the mysteries of the Void Century. Everybody is searching for the titular One Piece, a great treasure left behind by the Pirate King, Gold Roger, on the last island of the Grand Line, but nobody seems to actually know what it IS. One Piece has another In-Universe example.