In the exact same way: You do not need to actively mitigate. In fact, we do not even HAVE much in the way of active mitigation. Tank stance is passive and most of your other mitigation tools are on CDs of 1,5 minutes and up, all of which are fire and forget buttons. Likewise, aggro is trivial to gain and hold to the point that it's near impossible to lose if you aren't focusing on DPS for extended periods of time.
The already mentioned healer power also plays a role: In part, you do not need that tank stance or mitigation because healers can heal you just fine without as long as you use your cooldowns when the tank buster hits. And STILL have downtime and mana to spare despite that.
And it comes with another critical issue: Hard enrages. When healers are weak, you do not need enrages, because every fight has the natural enrage of your healers going OOM. This in turn allows for a synergistic play between all roles: Better DPS will shorten the fight and thus allow healers and tanks more margin of error, as they don't need to endure as long with their given resources, while better tanks/healers allow the DPS more margin of error by prolonging the fight. With hard enrages however, the time tanks and healers can give the DPS has a hard limit. Their DPS however makes it easier to beat the enrage. Result? Healers and Tanks dealing damage is emphasized even further.
So why not just remove hard enrages? Because then tanks and healers could drag the fight on forever and cheese it. They have the power. Worse, damage dealers would no longer be needed, as tanks and healers are still "able" to do the damage themselves.
They simply made tanking and healing too simple and tanks and healers too powerful. That is why the issue is so very pronounced in this game. And it can be fixed, too. It requires some hefty reworks, but it can be fixed. But not by asking people on the forums, that'll have to go over game design.