Because tanks would be USELESS! When you are team fighting, who is the prime target to focus? THATS RIGHT! THE CARRY! Who does your team USUALLY focus? THATS RIGHT! THE TANK! Do you know why? Not because they are stupid piles of , well sometimes that's why lol but usually its because if they are not positioned correctly, the only way they will survive is if they kill the tank before the tank kills them. Now if tanks did basically no damage, there would be no reason whatsoever to target them. If they did 0 damage, the only way for ANY tank to be useful would to either have so much cc to reliably block off the enemy team with multiple dashes, teleports, and spell shields from your carry, OR to have a mega buffed knights vow on every tank to mitigate the damage being pumped out by assassins onto your carry, making the carry in itself basically a tank.
Honestly, its kind of dumb, I'm not gunna lie. It sometimes (all the time) allows tanks to either abuse their kit in dumb ways cough (look at this things -->
<-- ad ratio on his q for a huge mind blow) cough or just go off their role as a tank while still building tank and carry a game. This is mostly a problem in the trash elo me and most of the league community resides in as me and others don't know how to position for a team fight for sh. Oh and if tanks did no damage no one would play them. That's my 2 cents.