You have to master your rotation so thoroughly that you can spend your time looking at your feet rather than the mob you’re killing, so you can dodge any mechanic that’s about to land on you.
You have to have a good nameplate addon (barring DBM, which is better) that clearly shows when mobs are casting. You have to memorize what those casts do, and do your best to pop a defensive or interrupt/stun them before they go off.
You need to position yourself correctly or cleaves and frontals will one shot you.
You need to sometimes pre-emptively use defensives during large pulls, because two uninterrupted mob abilities can sometimes unluckily target you, and it’s not worth the loss in DPS to die to something that’s sometimes a draw of the lottery.
You need to do all of this while moving. And staring at your character’s feet.
Anybody saying it’s “easy” is wrong. It’s easy for me now, because I’ve built muscle memory.
And that’s not even mentioning the value of having a good monitor, good graphics card, good internet connection, good mousepad, mouse, keyboard etc. When I had a crappy system (bad FPS, cheap and overly sensitive mouse, uncomfortable keyboard) my DPS was far worse due to slower response times.
And that’s not even mentioning the most important factor: your total and utter focus. Have a roommate blasting loud music or talking to you while you’re pushing content? Congrats, you will die eventually. Is your phone ringing in a distracting way? That two seconds of losing focus is enough to kill you once and put you at the absolute bottom of the meter.
I have seven 70s, all 440+ ilvl and 2.4k io or higher. My main is just shy of 3k io. I have an outside life too, but I make a commitment to be the best I can at whatever content I sign up for.