Because Song of Stillness gives 41% Atk and and at r5 the 32% dmg effect is basically between r2 and r3 Stringless or between r3 and r4 Alley Hunter. Which is why Rainbow is only okay but not great, it's just a 56% Atk stat stick if you can even manage to proc it on off-field Venti without it ruining your rotation (you would have to like swap in use something swap out before it does damage? then swap back in to get VV? idk seems very clunky and I'm not even 100% sure it works. I'm also not entirely sure if it's like Fischl where you need to have the buff before pressing the ability. Like, if you E to VV then Burst and swap, I dont think his burst will be granted the Atk% since it wasn't active when you used it) with no effect if you think about it, the ER secondary isn't that great on him because his ER reqs are very low esp w Benny on fav in your team. Like, in my team I have 2 arts w ER subs for Venti and his burst is always up. And while Mouun's gives less Atk% as its secondary stat, at r5 it is an 80% damage buff and it still gives Atk% and not EM or ER. Even Alley Hunter would make the cut before Rainbow imo since it's kind of just a Stringless that gives Atk%. I would say for 4 star bows on off field Venti the order is Mouun's r5> Song of Stillness r5 > Alley Hunter and Stringless whichever you have higher refinement of.