Why do tanks do more damage early game than pure damage dealers, riot?

I played as Talon top lane to a Tahm Kench. At the beginning of the game, we had decent trades. Non of us had died, i had more farm and experience by level 6. At first back, Tahm Kench goes for straight armour to counter my damage, in which I buy some sustain. I have Tiamat by 10 minutes into the game, and he is building into a Sunfire Cape. At this point all I am trying to do is poke him with my W and get him low enough to pop his shield. He has roughly 500 hp at this point and I'm using Corruption Potion to regenerate some mana back. He has two stacks on me at this point, he then proceeds to flash over the minions, use is tongue to slow me then auto me one more time. I have 500 health at this point. He then eats me, dealing roughly 200 or so damage, causing me to W, ult, and run away. He still has roughly 300 hp. I don't understand why this champion, building only armour, can do significant amounts of damage. Again, he has ZERO damage items and yet he does SO MUCH damage. Riot, you clearly don't understand what a tank is. Because tanks don't do damage. It makes no sense. I poked him down, I stayed clear of him, and yet for no reason at all, he still does incredible amounts of damage. Fix this riot. Prove to us you can do something right.