Because spellvamp was a pure stat not limited like Death's Dance is against AoE, where most of the physical damage abilities you would even want to heal off are almost all AoE. IDK about Elixir. 15% effectiveness for spellvamp on AOE would make it complete garbage.
I get lifesteal might feel bad to play against, but they still heal back in segments.
Take the Ryze rework for example with spellvamp ultimate. He'd activate it and murder just about everyone around him while simultaneously healing almost his entire health pool in each mini-rotation of resets because of his burst, but even more so because of the fact he could reset most of his burst and then do it again.
In particular, mage abilities individually all hit far harder than any one ADC autoattack lategame. That's the main problem.
An ADC's lifesteal is split up over their individual autoattacks.
An ADC might heal let's say, around 160-200 health an auto lategame (assuming around 20-20+% lifesteal with 800 damage autos).
Let's take… Annie for example. With ult up lategame and around 800 AP, she can drop 2710 damage near instantly against just one squishy. If her W or R hits more, that number skyrockets even higher. In teamfight settings, her W can easily hit 2-3 people, and her ultimate can hit all 5 without too much difficulty.
Let's say it's 2 tanks, 3 squishies. Fairly standard.
The W would probably hit the two frontline tanks + a squishy. That's another 941 damage to the two tanks to add to the 2710 total. I'm placing tanks at 150 MR, Annie has Void Staff.
The R would also hit the other 2 squishies + 2 tanks. That's another 1840 for the 2 squishies and then around 931 damage to the two tanks.
2710+941+1840+931=6422 magic damage dealt through resists and all.
20% spellvamp of that is almost 1285 health healed. In a split second.
On a champion who would probably have no more than around 1900-2000 health maximum lategame, just slightly more than an ADC most likely.
1285/1950 is almost 66% health healed almost instantaneously.
180/1800 is about 9-10% health healed against a squishy target per auto. You're not going to get off more than around 2 autos in the amount of time Annie could nuke a team.
That's the problem.
Sure, Annie's healing is pretty much limited to Q and W only afterwards each of which can do around 800+ damage similar to an ADC but with a lot longer cooldown.
But between the AoE burst damage and her massive instantaneous heal without even counting the CC versus healing broken up into segments that can be stopped to stop said healing flow, which seems like the bigger problem to you?
You're going to have a hard time even damaging Annie before she jumps in with her AoE team nuke. Any potential damage you would have done would also be instantly erased afterwards.
Far less time to react to mage dropping entire rotation in a split second versus an ADC who continuously heals up over time in small segments.
Phantom Dancer and DD on ADCs are problems sure.
But that's next to irrelevant to the health gain from damage issue itself. They would be problems regardless with their power.
You can't really stop a mage from just blasting an entire team with everything lategame instantly outside of flanking and gibbing them before they do it, because they'll stay at a safe long range. CCing the mage after is basically pointless.
If you CC the ADC though who has to actively keep attacking over time for their heal, their healing flow which is a lot slower but potentially longer gets cut off right then and there. If it weren't for Death's Dance and PD being stupid broken right now, that'd be the end of it. Bye ADC, understand?
TL;DR - lifesteal exists while spellvamp doesn't because there's more potential counterplay than with spellvamp, and PD/DD are the issues making lifesteal even stronger, not lifesteal in of itself