Tanks and Healers do a ton of damage, no doubt.

But they kinda need to. Particularly during boss fights, where healing and active mitigation are rarely a necessity.

Damage being taken is relatively predictable, meaning Tanks normally pop CDs only when necessary and healers have on average a 70% uptime of DPSing during fights (Stone IV...Stone IV....Stone IV....are you hurt yet!?).

However, trying to tip the balance too far in the other direction creates the WoW problem, where if a healer's resource pool is too low or a tank's not generating their resource fast enough, you can easily die while trying to restore your resource. Damage is extremely constant and rapid in WoW, to the point that much of a Tank's active mitigation is baked into their offensive rotation, with a few additional tools that aren't. Healers are pretty much the opposite of ours (I played a Holy Priest for most of Legion and the beginning of BfA), where they have 30% DPS uptime, if that. Usually, I'd average it even lower.