Is it possible that a WHM can out DPS a tank? Yes but only in fringe circumstances where the WHM is playing the absolute best they can, where they don't heal much at all.
And those fringe cases are generally accepted to be outliers and shouldn't be used to balance jobs. Because in average cases of healer vs tank damage, tank wins every time.
Again, if you're concerned about DPS numbers, play a DPS, the primary role assigned to do damage.
Average DPS numbers
^^ That box in the middle is what you need to be looking at, not the very far right edge of healers compared to the very far left edge of tanks.
However, ultimately it's up to SE and they've decided that they don't want tanks and healers to be large contributors to raid damage.
Do I agree with it? As a healer No. I'd love to have more complex things to do during downtime that just 2111111111111. Be grateful that's not your damage rotation from lvl 4 to lvl 80. Do I have to live with it? As a player playing their games, Yes.