People say that tanks are low in damage, but they're only half right...
What they don't get is that tanks are, by design, low in SINGLE TARGET damage. But they're great in dealing AoE. And for good reasons too, just think about it.
What are Tanks used for? They are high CC melee ranged "mages", they works best when paired with others. Where do you find the most allies at once? In a team fight. What's the most efficient type of damage in a team fight? AoE. Therefore, AoE is the best type of damage for a tank to have. (Singed poison trail, Cho'Gath QWE, Rammus Powerball and Tremors, Maokai QER, Nautalis WER, Leona WER, etc etc.)