Why is Arno never told about Shay?

Just finished AC Rogue and to be honest, i liked it. Shay’s motives for turning to the templars are kind of hypocritical and the narrative (at times) lacks common sense but it was a fun game.

But one question i couldn’t help but keep asking was Why did Arno and Shay never fight or even acknowledge each others existence? in Unity when Arno is recruited you can see in his Dream sequence how charles dies and it’s nothing like how shay calmly assassinates him in Rogue, Meaning Arno was never actually told about Shay and how his father was really killed.

You could chalk this up to “He was raised by templars so they may have hid it from him” but regardless it’s such a wasted opportunity. Shay is a bad-ass templar with multiple weapons at his disposal, Even in his 60s i’m sure Shay could’ve been worked into the Game (Or a DLC replacing that stinker French Revolution DLC)

Also after playing Unity and Rogue i’d have to say Prime Shay would handle Arno pretty easily but 60+ Year old Shay vs Arno is up for debate