Simple explanation really. It's an oversight, because only these skills have a cap that needs to be calculated separately. I am pretty sure the instant damage portion at the start already has a hybrid scaling. You could also ask why the stamina morph still has minor breach on it when all physical damage has a chance to apply minor breach through its status effect.

Come to think of it, I'm not sure if sorcerer pets scale of max stamina yet. The one that costs stamina and deals physical damage maybe but the rest probably don't. And there are still mundus stones that only give weapon damage or spell damage, not both.