Because a shitton of armor does nothing if you don't pair it with health.
And health is more valuable a defense early-game anyways (unless you're top against a strictly AD/AP champion), simply because nobody has a special penetration to it then.
Randuin's and Thornmail have basically no HP between them. However, toss in a Warmog's, and you suddenly become much harder to kill.
Also, ADCs are kinda meant to kill tanks. A tank's job is to survive and lock down bursty targets (which they do very well at - even having one armor item that isn't boots will let you do that fairly well). When you're a tank, you're meant to do two things, one of which you will be better at:
- Engage onto the enemy team. Disrupt them, lock them down, pull them to you, whatever it is, do it.
- Protect the squishies, especially the immobile ones. Use CC, bodyblock attacks, or fucking eat the squishy, whatever it is you do, do it. You're not quite meant to protect assassins or fighters as much, given that you can't really interpose yourself between the enemy team and them when they're trying to be on top of the enemy team.
Note that neither of these are "survive 10+ seconds of 2+ DPS champions hitting you". Items like Randuin's are meant to help you survive a bit longer against DPS, but by no means is it meant to be an auto-win against them. Thornmail is designed to counteract lifesteal, so that the enemy ADC can't use you as a walking battery pack to heal damage.
If you're still having trouble, try rushing
,
, or even
, and pair that with Overgrowth. Make your second item a more armor-oriented item. Try to total at least 800 health from items before going into something like
or
(Although you should still rush a
into champions like Fiora, of course). Also, pairing a lot of AA-reduction items together is somewhat inefficient. Unless you're laning against Yasuo or Tryndamere, you don't need
and
, and you should definitely not get them all in a row. A
should usually be your first item, there.