A question about feast or famine on champions, specifically when it comes to healing

So my question is this: Why do we gut/send to the dumpster champions with absurd healing/sustain like and , where their sustain tends to be based off of how much damage they deal to the enemy, when we allow lifesteal, which is basically the same thing, to exist in its current state?

You could argue that it is not the same thing, since with and it is an innate part of their kits, and that lifesteal requires investment into something like a or , but now that we have Warlord's Bloodlust, that argument is much weaker, since you could take it in every game for lifesteal you spent no gold on getting. However, with lifesteal, unless you attack, you do not get healing. And unless your attacks powerful, you won't heal much at all. So doesn't that lead to the same feast or famine problem that I listed above with and ? These champs need to deal damage to heal/shield, and lifesteal users must deal damage to heal, the only difference being that with one it is a part of their kit, and with the other it is a mastery that is taken on ADCs around 75% of the time.

ADCs usually cannot make it to late game right now due to the bot lane dive meta, but when they do I'm fine with their damage, it's just the healing off of that damage that's disgusts me.

I just want to know why these cases aren't treated in the same way.