All champions have base damage and resistances/health.
Tanks build a lot of defensive items to improve their ability to survive damage.
Damage-dealers build many offensive items to improve their ability to deal damage.
Why do you expect to be able to survive against a tank's damage without any defensive items? While they haven't built much in the way of offense, you haven't built much in the way of defense either.
The only reason tanks appear to deal similar damage as damage-dealers is because:
- damage-dealers typically have to damage the tank, who takes reduced damage
- tanks typically damage enemy damage-dealers, who have little defense
If you've ever watched two tanks try and kill each other (especially if they have some form of sustain), you would know that their actual damage is quite low in comparison to damage-dealers.