why does spell vamp do 1/3 healing on aoe?

Think about it. If spell vamp were strong, too many mages would build it. God knows no one wants to see a Veigar ult someone and get 1000 health back, but for champions that normally build spell vamp, their damage is mostly multi-targeted spells. And while multi-targeted spells can potentially deal more damage than single target abilities (depending on the champ and how much damage they can do), it feels extremely lackluster. When a Mordekaiser uses his abilities to clear a minion wave he feels a little cheated to see that his abilities cost him more health than his spell vamp item(s) healed, considering life steal exists and lets AD champs clear and heal and even other mages that can just expend mana and come out with more health through regen. And while this is a specific case and one could argue exemplifies Mordekaiser's imbalance, I'm really trying to build an argument for the mechanics of spell vamp.

The way I see it, there's no real advantage to have spell vamp perform differently for aoe and single target abilities. It creates a weird discrepancy between expectations and outcomes. As I look through the champion roster I see maybe 5 champions that are without a multi-target damaging ability (WW, poppy, trundle, maybe shaco, and vayne).

My point is, why make spell vamp heal for 1/3 damage off of 12%...essentially 4%, just because of different cases. Unless, and I don't have the data obviously, the aoe healing is just incredibly strong at higher percentages. Otherwise I think lowering the percentage, down to what, 7% on revolver? 10% on WoTA? and removing the reduced effect on aoe makes for a more clear design without being overly powerful. You reduce the healing from single target spells rendering spell vamp still undesirable for most mages, meaning Veigar ults won't be preposterous, while giving a little more love to champs like morde and vlad that use health as a resource.