Why do AD Champions get to build Lifesteal and regain huge chunks of health in burst
While dedicated healers are constrained by cooldowns and/or significant costs, and at best can bolster their sustain with Spirit Visage?
Swain's healing costs an increasing amount of mana per second, and heals him in DoT increments, so you can still out-burst it. Vladimir's primary healing is CD based and becomes weaker the higher his health is, as well as the cd being telegraphed by his resource bar. Dr. Mundo's healing is gated around the CD of his ultimate, and is otherwise slow and trickling with his passive. Fiddlesticks has to stand still and hope that his target stays within the tether range. Soraka sacrifices her own health to heal an ally. Zac has to pick up blobs that he drops by casting abilities - using up health in the process.
Ignoring that a lot of these Champions lose a huge amount of effectiveness thanks to a single, easily accessible stat - a core item on nearly every mana mage if you're AP, or a measly 800g.
These are Champions that are inherently toxic and unfun to play against, apparently.
Meanwhile, any ADC with a Bloodthirster and a Runaans can heal themselves from nothing to full health in 3-4 autoattacks.
Mmhm.
then do any damage.
is one of the most expensive items per it's cost to actually build, at a cost of 35 gold per stat, only beat out by critical strike chance 
at 40 gold per stat, (which again, requires AD to make use of) and life steal 
which is 37.5 gold per percent of lifesteal. In order to make a very good use of this, we will say roughly half of life steal building ADs need to buy attack speed 
, which is the third most expensive stat per gold.
at 37.33 gold per stat, and % movement speed 
at 39.5 gold per stat. I discount these because % move speed and flat magic pen aren't super prevalent on a lot of items.
which has a value of 20g per stat because if we do less damage, we will naturally heal for less, as life steal returns a % of the damage that you do after resistances. As we want to attack quickly, and not with slow heavy autos, the health comes in at a fairly steady rate, with the risk of being in range of many gap closers on a build that cannot really afford to build any kind of resistances itself as it needs 
at a lower price of 22.5 per stat for good measure for dealing with tanks, despite flat armor pen being worse than % from last whisper, as the % from last whisper is harder to calculate efficiently for it's raw value.
as they have some way of abusing the synergy through free stats or on hit effects.
, 
, 
, 
, 
, and to a less bruiser degree and more tank degree 
. All of these give him naturally tankier stats which allow him to make use of the free healing in his kit, while making him harder him to kill.
, as well as ability power to strengthen what is actually burst healing, as it does not come in at a truly steady rate like life steal. It is also important to remember that healers like 
don't need to actually get into range of any enemy champions to be effective with their healing. Some have abilties that are more useful when being aggressive, but are also viable being used for entirely defensive purposes.
counter life steal.
BRO!
