User blog comment:StrategicMagic/Carry by Proxy - Why (Some) Assasins Don't Fall Off Late/@comment-188.193.164.5-20121208222626

long text short.

armor/resistances make you more tanky.

but LB falls off cuz she deals ONLY magic dmg, so its easier to tank her out, same with riven (armor instead of MR).

kogmaw is a monster cause he deals huge AD dmg while passively hitting you with max%HP magic damage, making it an "either/or" decision when buying defensive stats, or having less of both, which still makes him deal alot of "hybrid" damage.

thats why he doesnt fall off.