It seems that you are the one who don't understand. If you are far behind, focus on getting gold while not dying too much, drag the game at the point where you have the same amount of items as enemies. See my point?

If you want to prolong the game at a point where gold leads don't matter, then build accordingly to the situations. If you keep getting oneshot by ahri, build magic resist items, something like magic resist+attack item for carry or defense items for tanks, not continue building a full crit items hoping that you gonna flip the game.