About lifesteal: Why don't they give it the same treatment they gave to Wota's spellvamp?

Nerf lifesteal on items (including Death's Dance which is technically not lifesteal). However, make lifesteal ignore armor. Now, lifesteal can be better balanced by not making it suck against armor stackers (if you don't have armor penetration) and not be opressive against squishies. It also makes it so that you can better know how much a character will heal with their auto-attacks. It is hard to estimate how much one will heal with lifesteal because it is reduced by armor, including base armor, so you are in pratice healing less than you should unless you have armor penetration. It means that lifesteal has to be high so that it is decent against armor stackers, but this makes it opressive against squishies that can't instantly 100-0 you.

In the past, Riot changed WotA for that exact reason. Back then, the spellvamp was higher, but because it directly scaled with the damage dealt, magic resistance would reduce it. This allowed you, for example, to counter Vladimir by building a single magic resistance item. Not only this would make you resist his damage, but also reduce his sustain. This made it more difficult to balance Vlad, as he had to have higher healing than usual, which would make him opressive if ahead, or against squishies. Champions with natural healing on their kits would also face the same problem. Examples include Renekton and Swain. Riot made it so that their sustain, instead of scaling with the damage they dealt, it would scale off a separate value. In case of WotA, they made it so that the spellvamp ignored magic resistance, then they reduced the spellvamp to compensate. Boom. Now Vlad could still sustain against opressive matchups but now he wouldn't be opressive himself if the opponent couldn't afford to build magic resistance or if he was ahead.