Buying AD Defensive items is useless

I remember this one time when I said that I got one shot by x champion and this champion and I was told many times that, that is why you buy defensive items. Well after some time of playing league and trying out the defensive items it feels useless, sure you can get these items but you will still die thanks to lethality completely ignoring armor and items that provide loads of magic penetration and because you would purchase these items you would loose a lot of your damage and essentially be useless in team fights since ADC's are most useful stat is Critical Strike.