Well, when you're at the point in the game where you've finished 4+ major items, almost anyone will 2 shot a carry. AP carries will look at you and you'll die. Bruisers will slap you with their combo and you'll die. Even fulltank malphite should be able to ult+E+Q to nearly instantly kill you. So stacking life leech is kinda weird. Either you're not under attack and you don't need it, or you take some damage but survive, in which case you don't need a lot to heal back up, or you're under focused attack and you'll die anyway.
Maybe it's useful for adc duels.