Most can't be bothered to do the former very well either.
Granted part of it is just the game not wanting to hurt anyone's feelings, so nothing indicates to the DPS player that their freestyle rotation is doing less damage than the tank.
DPS responsibility is the inverse of tanks.
They have close to none in MSQ content, dungeons, normal raids, trials or alliance raids where you can do abysmal damage and the boss will fall over eventually anyway.
They only start to have responsibility for the party's success in extreme trials, but then it gets exponentially higher as you go up in content difficulty, where both their damage output and additional party mitigation become a requirement.