Erm if you are against a tanky team you will deal less damage yourself. Damage dealt also heavily depends on how often you are able to attack a champion. Tanks can always go in and attack because they don't get 1 shot.
Its really getting old how players misinterpret the damage number over and over again in hundreds of hundreds of threads. How do you even jump to such a conclusion, are you just copying salty youtubers that tell you "tanks are op because they deal too much damage" or do you look at the statistics after a match like " Ok, what do i blame now except myself for losing this, oh the tank dealt the most damage, I take that"?