User blog comment:Berryeater/League of Rework, Part 2, attributes/@comment-27080952-20160526040527/@comment-26378838-20160526141839

Sorry, but we don't have spell vamp on items anymore, we have similar effects but they are clearly defined for niches, thus they are much, much less inconsistent than spell vamp, which had a much wider user pool.

And just because YOU find DD better for sustain than lifesteal on your picks doesn't mean it applies to all possible cases, specially to marksmen who deal mixed damage. It also doesn't mean lifesteal is lesser than DD, because both are healthy, consistent mechanics in their specific jobs and you'd destroy DD's consistency by modifying its intended job.