Board Thread:Naruto Discussions/@comment-27135393-20160803143409/@comment-3279282-20160803184302

Because the series isn't about Shisui. We don't know when he joined, but definitely earlier than Itachi.

The Mangekyo information is from a novel and thus not manga-canon. We don't know how much older than Itachi he is.

People don't believe that because of the Kotoamatsukami cooldown, but because of the established timeline for those events. See here.