It's pretty much because of that bonus armor penetration he gets from his ultimate, and because Sheen items are a little ridiculous.
Someone suggested before to change the bonus armor pen on Last Breath to apply only to critical strikes (heavily encouraging a critical strike build).
Either that OR make his bonus armor penetration duration scale with how many champions get caught in his ultimate (in a sort of similar way Fiora's Grand Challenge works, scaling with how many vital's were hit). That way, he could stand to lose a lot of damage in 1v1/1v2 situations, and only deal good consistent damage if he lands a really good ultimate.