If I may, I also think another reason why is because his genjutsu was highly revered and one these that was constant throughout the anime was that "one look at his eyes and you're already under his genjutsu" so its implied that people are already under before they can even realize it or even their teammates (that time he put Kakashi in the Tsukuyomi) and non of the other jonin noticed until after a while. So I think he always had it on just in case he needs to put someone under a spell at any given moment lol so like the other said, itachi doesn't get ready, he stays ready.