Not 100% sure you are not trolling, asking for buffs for Zac and Vi...but they nerf things because just buffing alternatives turns into an arms race of buffing. You'd keep having to buff and buff, each time something showed itself as OP. Eventually the game wouldn't even resemble the one we have today.
Also you are forgetting that a lot of time it's not necessarily champions themselves that suddenly become OP. A lot of the time it's item changes, meta shifts, mastery changes (Looking at you CoC), and even jungle changes (KRUUUUGS) that cause champions to spring forth. So just buffing champs will never bring you to a balanced state unless you never change non champion factors ever again. Because eventually CoC would be made and even though Lee was balanced against Vi, suddenly Vi has an edge due to completely non-champion related reasons.