I am curious as to why spellvamp doesn't have ratios limiting its effectiveness on problem cases.

Imagine, for a second, if AP worked like spellvamp did. Where any ability that was single target, by the nature of being single target, had a 1.0 AP ratio, and every ability that could hit more than one target had a .33 AP ratio.

Vlad would destroy the game. As would any AP champion with a low cooldown single target ability. In response, you'd have to nerf AP to a point where the abusers are just barely kept in line. Thereafter any champion unlucky enough to only have AOE damage in their kits would be basically worthless, as they simply wouldn't have any way to remain relevant in game. Quite simply, everything would be a mess. And, naturally, to prevent this, the game has AP ratios. So that if Vlad, or any given AP champion scales too well with AP, you can nerf that instead of destroying the stat and keep balance throughout the entire game.

But spellvamp is in such a horrible position, wherein it's so powerful on a select few champions that it's all but useless on the rest of the cast. In the past this has caused a ton of difficulty creating spellvamp items, made it impossible to make any decent sources of AD spellvamp, and turned Vladimir into a monster multiple times. The solution, s'far as I see, is simple. Add spellvamp ratios to abilities. So that if ever Vlad becomes too powerful with the stat, you can nerf his spellvamp ratios. And if, say, Orianna is basically useless with it, you can buff hers. And e'er so often I find myself wondering why such a basic change hasn't been implemented yet.