Because they don't want to show Vian befriending Aarav bcoz of Katha
Otherwise it would've looked like scheme or shakunivanshi approach
I think it's better
In OG i always felt like that
keeping with Viaan's current arc, he won't even approach Aarav if he knew he was katha's son. Also, Aarav seems to be possessive of his mom, so he may automaitically dislike any male around her.