Board Thread:Theories and Speculation/@comment-34656247-20191119070420/@comment-1210805-20191122044715

Like Squinty said, Itachi probably thought "Madara" had one eye, as that's all he'd ever seen from him. Itachi didn't know about the stockpile of spare Sharingan, and probably believed that Madara survived his previous "death" by using Izanagi. As far as Itachi would have known, even if "Madara" used his remaining eye to escape the Amaterasu by casting Izanagi, he would have been left without his eyes, depriving him of his remaining Sharingan, and most importantly, his Mangekyo. Itachi's ultimate goal was to protect Sasuke and Konoha. Depriving Madara of his eyes would have gone a long way to achieving that.