Because ADC get more GPS(gold per second) than most tank and bruiser, and by the time tank/ bruiser get 5 item by default, ofcourse ADCs already have 6 items.
And with 3 items:
(have 220 AD) level 12 for example, taking , one aa to minion wave deal 550 damage to main target and 220 damage to each side target, = 990 damage, life steal = 25% lifesteal (3 from doran, 2 from mastery), that heal you for 247.5 heal.
warlord heal you for 154 heal, that for total around 400 heal.
have 1316 heal at level 12, if she ult, she gain 50 AD that give 191 more heal in AA, if include damage from runes and mastery, she will heal 600 heal in that one Auto attack. If she use Tumble, she gain 50% AD in prime target, giving 84 more heal, to a total close of 700 heal in one auto attack while building full damage.
If her rune have 6% lifesteal, she will heal 110 more heal, let make them to 800 heal.
REMINDER THAT IT IS NOT HER FULL BUILD, SHE ONLY HAVE 3 ITEM AND A DORRAN.
AND yes, healing 800 hp in level 12 in one shoot while have 1200 hp total is possible, and that not even hard to archive. If she use crit damage rune + lifesteal and 18/0/12 mastery she can even heal 1200 hp (full heal) in one crit ult Q shoot.